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                         MINNESOTA POWER & LIGHT COMPANY

                                       TO

                              THE BANK OF NEW YORK
                         (FORMERLY IRVING TRUST COMPANY)

                                       AND

                                 W.T. CUNNINGHAM

                   (SUCCESSOR TO RICHARD H. WEST, J.A. AUSTIN,
                     E.J. MCCABE, D.W. MAY AND J.A. VAUGHAN)


                                    AS TRUSTEES UNDER MINNESOTA POWER &
                                    LIGHT COMPANY'S MORTGAGE AND DEED OF
                                    TRUST DATED AS OF SEPTEMBER 1, 1945

                             ------------------------


                        TWENTIETH SUPPLEMENTAL INDENTURE

                        PROVIDING AMONG OTHER THINGS FOR

            FIRST MORTGAGE BONDS, 6.68% SERIES DUE NOVEMBER 15, 2007

                              (TWENTY-SIXTH SERIES)


                          DATED AS OF NOVEMBER 1, 1997



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                        TWENTIETH SUPPLEMENTAL INDENTURE

     THIS  INDENTURE,  dated as of  November 1, 1997,  by and between  MINNESOTA
POWER & LIGHT  COMPANY,  a  corporation  of the State of  Minnesota,  whose post
office address is 30 West Superior Street, Duluth,  Minnesota 55802 (hereinafter
sometimes called the "Company"), and THE BANK OF NEW YORK (formerly Irving Trust
Company),  a corporation of the State of New York,  whose post office address is
101 Barclay Street, New York, New York 10286  (hereinafter  sometimes called the
"Corporate Trustee"),  and W. T. CUNNINGHAM (successor to Richard H. West, J. A.
Austin, E. J. McCabe, D. W. May and J. A. Vaughan), whose post office address is
3 Arlington  Drive,  Denville,  New Jersey  07834 (said W. T.  Cunningham  being
hereinafter  sometimes called the "Co-Trustee" and the Corporate Trustee and the
Co-Trustee being  hereinafter  together  sometimes  called the  "Trustees"),  as
Trustees  under the Mortgage  and Deed of Trust,  dated as of September 1, 1945,
between the Company and Irving Trust  Company and Richard H. West,  as Trustees,
securing  bonds  issued  and  to be  issued  as  provided  therein  (hereinafter
sometimes  called the  "Mortgage"),  reference to which mortgage is hereby made,
this  indenture  (hereinafter  sometimes  called  the  "Twentieth   Supplemental
Indenture") being supplemental thereto:

     WHEREAS, the Mortgage was filed and recorded in various official records in
the State of Minnesota; and

     WHEREAS,  an  instrument,  dated as of October 16,  1957,  was executed and
delivered under which J.A. Austin succeeded  Richard H. West as Co-Trustee under
the Mortgage,  and such  instrument  was filed and recorded in various  official
records in the State of Minnesota; and

     WHEREAS,  an  instrument,  dated as of  April 4,  1967,  was  executed  and
delivered  under which E. J. McCabe in turn succeeded J.A.  Austin as Co-Trustee
under the  Mortgage,  and such  instrument  was filed and  recorded  in  various
official records in the State of Minnesota; and

     WHEREAS,  under the  Sixth  Supplemental  Indenture,  dated as of August 1,
1975, to which  reference is hereinafter  made, D.W. May in turn succeeded E. J.
McCabe as Co-Trustee under the Mortgage; and

     WHEREAS,  an  instrument,  dated  as of June 25,  1984,  was  executed  and
delivered  under which J. A. Vaughan in turn  succeeded  D.W. May as  Co-Trustee
under the  Mortgage,  and such  instrument  was filed and  recorded  in  various
official records in the State of Minnesota; and

     WHEREAS,  an  instrument,  dated  as of July 27,  1988,  was  executed  and
delivered  under  which W. T.  Cunningham  in turn  succeeded  J.A.  Vaughan  as
Co-Trustee  under the Mortgage,  and such  instrument  was filed and recorded in
various official records in the State of Minnesota; and



                                       -2-


     WHEREAS, by the Mortgage the Company  covenanted,  among other things, that
it would execute and deliver such supplemental  indenture or indentures and such
further  instruments and do such further acts as might be necessary or proper to
carry out more  effectually  the purposes of the Mortgage and to make subject to
the lien of the  Mortgage any  property  thereafter  acquired and intended to be
subject to the lien thereof; and

     WHEREAS,  for  said  purposes,  among  others,  the  Company  executed  and
delivered the following indentures supplemental to the Mortgage:

              DESIGNATION                                    DATED AS OF
              -----------                                    -----------
       First Supplemental Indenture ...............          March 1, 1949
       Second Supplemental Indenture...............          July 1, 1951
       Third Supplemental Indenture ...............          March 1, 1957
       Fourth Supplemental Indenture ..............          January 1, 1968
       Fifth Supplemental Indenture................          April 1, 1971
       Sixth Supplemental Indenture ...............          August 1, 1975
       Seventh Supplemental Indenture .............          September 1, 1976
       Eighth Supplemental Indenture...............          September 1, 1977
       Ninth Supplemental Indenture ...............          April 1, 1978
       Tenth Supplemental Indenture................          August 1, 1978
       Eleventh Supplemental Indenture.............          December 1, 1982
       Twelfth Supplemental Indenture..............          April 1, 1987
       Thirteenth Supplemental Indenture...........          March 1, 1992
       Fourteenth Supplemental Indenture...........          June 1, 1992
       Fifteenth Supplemental Indenture............          July 1, 1992
       Sixteenth Supplemental Indenture............          July 1, 1992
       Seventeenth Supplemental Indenture..........          February 1, 1993
       Eighteenth Supplemental Indenture...........          July 1, 1993

which  supplemental  indentures  were filed and  recorded  in  various  official
records in the State of Minnesota; and

     WHEREAS,  for said  purposes,  among others,  the Company also executed and
delivered a  Nineteenth  Supplemental  Indenture,  dated as of February 1, 1997,
which  was  filed and  recorded  in  various  official  records  in the State of
Minnesota for which recording information is not yet available.



                                       -3-


     WHEREAS,  the  Company  has  heretofore  issued,  in  accordance  with  the
provisions of the Mortgage, as heretofore supplemented,  the following series of
First Mortgage Bonds:

                                             PRINCIPAL         PRINCIPAL
                                              AMOUNT            AMOUNT
SERIES                                        ISSUED          OUTSTANDING
- - - ------                                       ---------        -----------

3-1/8% Series due 1975....................  $26,000,000          None
3-1/8% Series due 1979....................    4,000,000          None
3-5/8% Series due 1981....................   10,000,000          None
4-3/4% Series due 1987....................   12,000,000          None
6-1/2% Series due 1998....................   18,000,000      $18,000,000
8-1/8% Series due 2001....................   23,000,000          None
10-1/2% Series due 2005...................   35,000,000          None
8.70% Series due 2006.....................   35,000,000          None
8.35% Series due 2007.....................   50,000,000          None
9-1/4% Series due 2008....................   50,000,000          None
Pollution Control Series A................  111,000,000          None
Industrial Development Series A...........   $2,500,000          None
Industrial Development Series B...........    1,800,000          None
Industrial Development Series C...........    1,150,000          None
Pollution Control Series B................   13,500,000          None
Pollution Control Series C................    2,000,000          None
Pollution Control Series D................    3,600,000        3,600,000
7-3/4% Series due 1994....................   55,000,000          None
7-3/8% Series due March 1, 1997...........   60,000,000          None
7-3/4% Series due June 1, 2007............   55,000,000       55,000,000
7-1/2% Series due August 1, 2007..........   35,000,000       35,000,000
Pollution Control Series E................  111,000,000      111,000,000
7% Series due March 1, 2008...............   50,000,000       50,000,000
6-1/4% Series due July 1, 2003............   25,000,000       25,000,000
7% Series due February 15, 2007...........   60,000,000       60,000,000

which bonds are also  hereinafter  sometimes  called bonds of the First  through
Twenty-fifth Series, respectively; and

     WHEREAS, Section 8 of the Mortgage provides that the form of each series of
bonds  (other  than the First  Series)  issued  thereunder  and of coupons to be
attached to coupon bonds of such series shall be  established  by  Resolution of
the Board of  Directors  of the  Company  and that the form of such  series,  as
established by said Board of Directors,  shall specify the descriptive  title of
the bonds and various other terms thereof, and may also contain such



                                       -4-


provisions not inconsistent  with the provisions of the Mortgage as the Board of
Directors may, in its  discretion,  cause to be inserted  therein  expressing or
referring  to the terms and  conditions  upon  which such bonds are to be issued
and/or secured under the Mortgage; and

     WHEREAS, Section 120 of the Mortgage provides, among other things, that any
power,  privilege  or right  expressly  or  impliedly  reserved to or in any way
conferred upon the Company by any provision of the Mortgage, whether such power,
privilege  or right is in any way  restricted  or is  unrestricted,  may (to the
extent  permitted  by law) be in  whole  or in part  waived  or  surrendered  or
subjected  to any  restriction  if at the  time  unrestricted  or to  additional
restriction  if already  restricted,  and the Company may enter into any further
covenants, limitations or restrictions for the benefit of any one or more series
of bonds  issued  thereunder,  or the Company may cure any  ambiguity  contained
therein,  or in any  supplemental  indenture,  or may  establish  the  terms and
provisions  of any  series  of  bonds  (other  than  said  First  Series)  by an
instrument in writing executed and acknowledged by the Company in such manner as
would be necessary  to entitle a  conveyance  of real estate to record in all of
the states in which any property at the time subject to the lien of the Mortgage
shall be situated; and

     WHEREAS,  the  Company  now  desires  to  create a new  series of bonds and
(pursuant  to the  provisions  of  Section  120 of the  Mortgage)  to add to its
covenants and agreements contained in the Mortgage, as heretofore  supplemented,
certain  other  covenants  and  agreements to be observed by it and to alter and
amend  in  certain  respects  the  covenants  and  provisions  contained  in the
Mortgage, as heretofore supplemented; and

     WHEREAS,  the  execution  and  delivery  by the  Company of this  Twentieth
Supplemental  Indenture,  and the terms of the bonds of the Twenty-sixth Series,
hereinafter  referred to, have been duly authorized by the Board of Directors of
the Company by appropriate resolutions of said Board of Directors;

     NOW, THEREFORE, THIS INDENTURE WITNESSETH:

     That the Company,  in consideration of the premises and of One Dollar to it
duly paid by the  Trustees  at or before the  ensealing  and  delivery  of these
presents, the receipt whereof is hereby acknowledged, and in further evidence of
assurance of the estate,  title and rights of the Trustees and in order  further
to secure the payment of both the principal of and interest and premium, if any,
on the  bonds  from  time to time  issued  under  the  Mortgage,  as  heretofore
supplemented, according to their tenor and effect and the performance of all the
provisions of the Mortgage (including any instruments  supplemental  thereto and
any  modification  made as in the Mortgage  provided) and of said bonds,  hereby
grants,  bargains,  sells, releases,  conveys,  assigns,  transfers,  mortgages,
pledges,  sets over and confirms (subject,  however,  to Excepted  Encumbrances)
unto THE BANK OF NEW YORK and W. T. CUNNINGHAM, as Trustees under the



                                       -5-


Mortgage,  and to their  successor  or  successors  in said  trust,  and to said
Trustees and their successors and assigns forever, all property,  real, personal
and mixed,  of the kind or nature  specifically  mentioned in the  Mortgage,  as
heretofore supplemented,  or of any other kind or nature acquired by the Company
after the date of the  execution  and delivery of the  Mortgage,  as  heretofore
supplemented (except any herein or in the Mortgage, as heretofore  supplemented,
expressly  excepted),  now owned or, subject to the provisions of subsection (I)
of Section 87 of the Mortgage,  hereafter  acquired by the Company (by purchase,
consolidation, merger, donation, construction, erection or in any other way) and
wheresoever situated, including (without in anywise limiting or impairing by the
enumeration  of the same the scope and intent of the foregoing or of any general
description contained in this Twentieth Supplemental Indenture) all lands, power
sites,  flowage rights,  water rights,  water locations,  water  appropriations,
ditches, flumes, reservoirs, reservoir sites, canals, raceways, dams, dam sites,
aqueducts, and all other rights or means for appropriating,  conveying,  storing
and supplying  water; all rights of way and roads; all plants for the generation
of electricity by steam, water and/or other power; all power houses, gas plants,
street  lighting  systems,  standards and other  equipment  incidental  thereto,
telephone, radio and television systems,  air-conditioning systems and equipment
incidental thereto, water works, water systems, steam heat and hot water plants,
substations,  lines, service and supply systems, bridges,  culverts, tracks, ice
or refrigeration plants and equipment,  offices,  buildings and other structures
and the equipment thereof; all machinery,  engines, boilers, dynamos,  electric,
gas and other machines,  regulators,  meters, transformers,  generators, motors,
electrical, gas and mechanical appliances,  conduits, cables, water, steam heat,
gas or other pipes,  gas mains and pipes,  service pipes,  fittings,  valves and
connections,  pole and transmission lines,  wires,  cables,  tools,  implements,
apparatus, furniture and chattels; all municipal and other franchises,  consents
or permits; all lines for the transmission and distribution of electric current,
gas, steam heat or water for any purpose including towers, poles, wires, cables,
pipes,  conduits,  ducts and all apparatus for use in connection therewith;  all
real  estate,  lands,  easements,  servitudes,  licenses,  permits,  franchises,
privileges,  rights of way and other rights in or relating to real estate or the
occupancy of the same and (except as herein or in the  Mortgage,  as  heretofore
supplemented,  expressly  excepted)  all the right,  title and  interest  of the
Company  in and to all  other  property  of any kind or nature  appertaining  to
and/or used and/or  occupied  and/or  enjoyed in  connection  with any  property
hereinbefore or in the Mortgage, as heretofore supplemented, described.

     TOGETHER WITH all and singular the tenements, hereditaments, prescriptions,
servitudes  and  appurtenances  belonging  or in  anywise  appertaining  to  the
aforesaid  property or any part  thereof,  with the  reversion  and  reversions,
remainder  and  remainders  and (subject to the  provisions of Section 57 of the
Mortgage) the tolls, rents,  revenues,  issues,  earnings,  income,  product and
profits  thereof,  and all the  estate,  right,  title  and  interest  and claim
whatsoever,  at law as well  as in  equity,  which  the  Company  now has or may
hereafter acquire in and to the aforesaid property and franchises and every part
and parcel thereof.




                                       -6-


     IT IS HEREBY  AGREED by the  Company  that,  subject to the  provisions  of
subsection  (I) of Section 87 of the  Mortgage,  all the property,  rights,  and
franchises  acquired  by  the  Company  (by  purchase,  consolidation,   merger,
donation,  construction,  erection  or in any other way) after the date  hereof,
except any herein or in the  Mortgage,  as  heretofore  supplemented,  expressly
excepted,  shall be and are as fully  granted  and  conveyed  hereby  and by the
Mortgage  and as  fully  embraced  within  the lien  hereof  and the lien of the
Mortgage  as if such  property,  rights  and  franchises  were now  owned by the
Company and were  specifically  described herein or in the Mortgage and conveyed
hereby or thereby.

     PROVIDED  that  the  following  are not and are not  intended  to be now or
hereafter granted, bargained, sold, released, conveyed,  assigned,  transferred,
mortgaged, hypothecated,  affected, pledged, set over or confirmed hereunder and
are hereby  expressly  excepted  from the lien and  operation of this  Twentieth
Supplemental Indenture and from the lien and operation of the Mortgage,  namely:
(1) cash,  shares  of  stock,  bonds,  notes  and  other  obligations  and other
securities not hereafter  specifically pledged,  paid,  deposited,  delivered or
held under the  Mortgage or  covenanted  so to be; (2)  merchandise,  equipment,
apparatus,  materials  or  supplies  held  for the  purpose  of  sale  or  other
disposition in the usual course of business; fuel, oil and similar materials and
supplies  consumable in the  operation of any of the  properties of the Company;
all aircraft, rolling stock, trolley coaches, buses, motor coaches,  automobiles
and other  vehicles and materials and supplies held for the purpose of repairing
or replacing (in whole or part) any of the same; all timber,  minerals,  mineral
rights and  royalties;  (3) bills,  notes and  accounts  receivable,  judgments,
demands and choses in action, and all contracts, leases and operating agreements
not  specifically  pledged  under  the  Mortgage  or  covenanted  so to be;  the
Company's  contractual  rights or other interest in or with respect to tires not
owned by the  Company;  (4) the last day of the term of any  lease or  leasehold
which may hereafter  become  subject to the lien of the  Mortgage;  (5) electric
energy,   gas,  steam,   ice,  and  other   materials  or  products   generated,
manufactured, produced or purchased by the Company for sale, distribution or use
in the ordinary course of its business;  and (6) the Company's franchise to be a
corporation;  provided, however, that the property and rights expressly excepted
from the lien and operation of this  Twentieth  Supplemental  Indenture and from
the lien and  operation  of the Mortgage in the above  subdivisions  (2) and (3)
shall (to the extent  permitted by law) cease to be so excepted in the event and
as of the date that  either or both of the  Trustees  or a  receiver  or trustee
shall enter upon and take  possession of the  Mortgaged and Pledged  Property in
the manner  provided in Article XIII of the Mortgage by reason of the occurrence
of a Default as defined in Section 65 thereof.

     TO HAVE AND TO HOLD all such properties, real, personal and mixed, granted,
bargained, sold, released, conveyed, assigned, transferred,  mortgaged, pledged,
set over or  confirmed by the Company as  aforesaid,  or intended so to be, unto
the Trustees and their successors and assigns forever.



                                       -7-


     IN TRUST  NEVERTHELESS,  for the  same  purposes  and upon the same  terms,
trusts and conditions and subject to and with the same provisos and covenants as
are set forth in the Mortgage,  as  supplemented,  this  Twentieth  Supplemental
Indenture being supplemental thereto.

     AND IT IS HEREBY COVENANTED by the Company that all the terms,  conditions,
provisos,  covenants and  provisions  contained in the  Mortgage,  as heretofore
supplemented,  shall affect and apply to the property hereinbefore described and
conveyed and to the estate,  rights,  obligations  and duties of the Company and
Trustees and the  beneficiaries of the trust with respect to said property,  and
to the  Trustees and their  successors  in the trust in the same manner and with
the same effect as if said property had been owned by the Company at the time of
the execution of the Mortgage, and had been specifically and at length described
in and  conveyed  to said  Trustees by the  Mortgage  as a part of the  property
therein stated to be conveyed.

     The Company further covenants and agrees to and with the Trustees and their
successors in said trust under the Mortgage as follows:


                               ARTICLE I
                     TWENTY-SIXTH SERIES OF BONDS

     SECTION 1. There shall be a series of bonds  designated  "6.68%  Series due
November 15, 2007" (herein sometimes referred to as the "Twenty-sixth  Series"),
each of which shall also bear the descriptive  title "First Mortgage Bond",  and
the form  thereof,  which shall be  established  by  Resolution  of the Board of
Directors of the Company,  shall contain suitable provisions with respect to the
matters hereinafter in this Section specified. Bonds of the Twenty- sixth Series
shall be dated as in Section 10 of the Mortgage provided, mature on November 15,
2007,  be issued as fully  registered  bonds in  denominations  of One  Thousand
Dollars and, at the option of the  Company,  in any multiple or multiples of One
Thousand  Dollars (the  exercise of such option to be evidenced by the execution
and delivery thereof) and bear interest at the rate of 6.68% per annum,  payable
semi-annually  on November 15 and May 15 of each year,  commencing May 15, 1998,
the  principal  of and interest on each said bond to be payable at the office or
agency of the Company in the Borough of Manhattan, The City of New York, in such
coin or  currency  of the United  States of America as at the time of payment is
legal tender for public and private debts.

     (I)  Bonds of the  Twenty-sixth  Series  shall not be  redeemable  prior to
maturity.

     (II) At the option of the registered  owner,  any bonds of the Twenty-sixth
Series,  upon surrender  thereof for cancellation at the office or agency of the
Company  in the  Borough of  Manhattan,  The City of New York,  together  with a
written instrument of transfer wherever




                                       -8-


required by the Company  duly  executed by the  registered  owner or by his duly
authorized  attorney,  shall  (subject  to the  provisions  of Section 12 of the
Mortgage) be exchangeable for a like aggregate  principal amount of bonds of the
same series of other authorized denominations.

     Bonds of the  Twenty-sixth  Series  shall be  transferable  (subject to the
provisions of Section 12 of the Mortgage) at the office or agency of the Company
in the Borough of Manhattan, The City of New York.

     Upon any  exchange  or transfer of bonds of the  Twenty-sixth  Series,  the
Company may make a charge  therefor  sufficient  to  reimburse it for any tax or
taxes or other  governmental  charge, as provided in Section 12 of the Mortgage,
but the Company hereby waives any right to make a charge in addition thereto for
any exchange or transfer of bonds of the Twenty-sixth Series.

     Upon  the  delivery  of this  Twentieth  Supplemental  Indenture  and  upon
compliance  with the  applicable  provisions of the Mortgage,  there shall be an
initial issue of bonds of the Twenty-  sixth Series for the aggregate  principal
amount of $20,000,000.


                              ARTICLE II

                           DIVIDEND COVENANT

     SECTION  2.  The  Company  covenants  and  agrees  that the  provisions  of
subdivision  (III) of Section 39 of the Mortgage,  which are to remain in effect
so long as any of the bonds of the First Series shall remain Outstanding,  shall
remain  in full  force and  effect  so long as any  bonds of the  First  through
Twenty-sixth Series shall remain Outstanding.


                              ARTICLE III

                       MISCELLANEOUS PROVISIONS

     SECTION 3. Section 126 of the Mortgage,  as heretofore  amended,  is hereby
further  amended by adding the words "and  November  15,  2007"  after the words
"February 15, 2007".

     SECTION  4.  Subject  to the  amendments  provided  for in  this  Twentieth
Supplemental  Indenture,  the  terms  defined  in the  Mortgage,  as  heretofore
supplemented,  shall, for all purposes of this Twentieth Supplemental Indenture,
have the meanings specified in the Mortgage, as heretofore supplemented.



                                       -9-


     SECTION 5. The holders of bonds of the Twenty-sixth Series consent that the
Company may, but shall not be obligated to, fix a record date for the purpose of
determining the holders of bonds of the Twenty-sixth  Series entitled to consent
to any amendment, supplement or waiver. If a record date is fixed, those persons
who were  holders at such record date (or their duly  designated  proxies),  and
only those persons,  shall be entitled to consent to such amendment,  supplement
or waiver or to revoke any consent previously given, whether or not such persons
continue to be holders after such record date. No such consent shall be valid or
effective for more than 90 days after such record date.

     SECTION 6. The Trustees hereby accept the trusts herein declared, provided,
created  or  supplemented  and  agree to  perform  the same  upon the  terms and
conditions herein and in the Mortgage set forth and upon the following terms and
conditions:

     The Trustees shall not be  responsible  in any manner  whatsoever for or in
respect of the validity or sufficiency of this Twentieth  Supplemental Indenture
or for or in respect of the recitals contained herein, all of which recitals are
made by the  Company  solely.  In  general,  each and every  term and  condition
contained in Article  XVII of the Mortgage  shall apply to and form part of this
Twentieth  Supplemental  Indenture with the same force and effect as if the same
were herein set forth in full with such omissions, variations and insertions, if
any, as may be  appropriate  to make the same conform to the  provisions of this
Twentieth Supplemental Indenture.

     SECTION 7.  Whenever in this  Twentieth  Supplemental  Indenture  any party
hereto is named or  referred  to,  this  shall,  subject  to the  provisions  of
Articles XVI and XVII of the Mortgage, as heretofore supplemented,  be deemed to
include the  successors  or assigns of such  party,  and all the  covenants  and
agreements in this Twentieth Supplemental Indenture contained by or on behalf of
the Company,  or by or on behalf of the Trustees  shall,  subject as  aforesaid,
bind and inure to the benefit of the  respective  successors and assigns of such
party whether so expressed or not.

     SECTION 8. Nothing in this Twentieth Supplemental  Indenture,  expressed or
implied,  is intended,  or shall be  construed,  to confer upon, or give to, any
person,  firm or  corporation,  other than the parties hereto and the holders of
the bonds and coupons  Outstanding  under the Mortgage,  any right,  remedy,  or
claim  under  or by  reason  of this  Twentieth  Supplemental  Indenture  or any
covenant,  condition,  stipulation,  promise or  agreement  hereof,  and all the
covenants, conditions,  stipulations,  promises and agreements in this Twentieth
Supplemental  Indenture  contained by and on behalf of the Company  shall be for
the sole and exclusive benefit of the parties hereto,  and of the holders of the
bonds and of the coupons Outstanding under the Mortgage.



                                      -10-


     SECTION 9. This  Twentieth  Supplemental  Indenture  shall be  executed  in
several counterparts,  each of which shall be an original and all of which shall
constitute but one and the same instrument.

     SECTION 10. The Company,  the  mortgagor  named  herein,  by its  execution
hereof acknowledges  receipt of a full, true and complete copy of this Twentieth
Supplemental Indenture.






                                      -11-


     IN  WITNESS  WHEREOF,  Minnesota  Power  & Light  Company  has  caused  its
corporate  name to be hereunto  affixed,  and this  instrument  to be signed and
sealed by its President or one of its Vice Presidents, and its corporate seal to
be attested by its Secretary or one of its Assistant  Secretaries for and in its
behalf,  and The Bank of New York has caused its  corporate  name to be hereunto
affixed,  and  this  instrument  to be  signed  and  sealed  by one of its  Vice
Presidents or one of its Assistant Vice  Presidents and its corporate seal to be
attested  by one of  its  Assistant  Treasurers  or  one of its  Assistant  Vice
Presidents, and W. T. Cunningham has hereunto set his hand and affixed his seal,
all in The City of New York, as of the day and year first above written.


                                           MINNESOTA POWER & LIGHT COMPANY
[MINNESOTA POWER & LIGHT COMPANY
        CORPORATE SEAL
          MINESOTA]                        By David G. Gartzke
                                              ----------------------------------
                                              David G. Gartzke
                                              Senior Vice President - Finance
                                               and Chief Financial Officer


Attest:

Philip R. Halverson
- - - ------------------------
Philip R. Halverson
Vice President, Counsel
 and Secretary




Executed,  sealed  and  delivered  by
MINNESOTA  POWER & LIGHT  COMPANY
in the presence of:


Lorie Skudstad
- - - -------------------------

Geraldine Peterson
- - - -------------------------




                                      -12-



                                         THE BANK OF NEW YORK
                                               as Trustee


                                         By Thomas B. Zakrzewski
                                            ------------------------------------
                                            Thomas B. Zakrzewski
                                            Assistant Vice President


Attest:

Remo J. Reale
- - - -------------------------
Remo J. Reale
Assistant Vice President


                                         W. T. Cunningham                 (L.S.)
                                         ---------------------------------
                                         W.T. Cunningham



Executed, sealed and delivered by
THE BANK OF NEW YORK AND W. T. CUNNINGHAM
in the presence of:


Michele L. Russo
- - - -------------------------

Essie Elcock
- - - -------------------------





                                      -13-


STATE OF MINNESOTA  )
                    )  SS.:
COUNTY OF ST. LOUIS )


     On this 20th day of November,  1997,  before me, a Notary Public within and
for said County,  personally  appeared DAVID G. GARTZKE and PHILIP R. HALVERSON,
to me personally  known, who, being each by me duly sworn, did say that they are
respectively the Senior Vice President - Finance and Chief Financial Officer and
the Vice  President,  General  Counsel and Secretary of MINNESOTA  POWER & LIGHT
COMPANY  of the  State of  Minnesota,  the  corporation  named in the  foregoing
instrument;  that the seal affixed to the foregoing  instrument is the corporate
seal of said  corporation;  that said instrument was signed and sealed in behalf
of said  corporation  by authority of its Board of Directors;  and said DAVID G.
GARTZKE and PHILIP R. HALVERSON  acknowledged said instrument to be the free act
and deed of said corporation.

     Personally  came  before me on this 20th day of  November,  1997,  DAVID G.
GARTZKE  to me known  to be the  Senior  Vice  President  -  Finance  and  Chief
Financial Officer and PHILIP R. HALVERSON, to me known to be the Vice President,
General  Counsel  and  Secretary,  of the above  named  MINNESOTA  POWER & LIGHT
COMPANY,  the  corporation   described  in  and  which  executed  the  foregoing
instrument,  and to me  personally  known to be the persons who as such officers
executed the foregoing  instrument  in the name and behalf of said  corporation,
who,  being by me duly sworn did depose  and say and  acknowledge  that they are
respectively the Senior Vice President - Finance and Chief Financial Officer and
the Vice President, General Counsel and Secretary of said corporation;  that the
seal affixed to said instrument is the corporate seal of said  corporation;  and
that they signed, sealed and delivered said instrument in the name and on behalf
of said corporation by authority of its Board of Directors and stockholders, and
said DAVID G. GARTZKE and PHILIP R. HALVERSON then and there  acknowledged  said
instrument  to be the  free  act and  deed of said  corporation  and  that  such
corporation executed the same.

     On the 20th day of  November,  1997,  before me  personally  came  DAVID G.
GARTZKE and PHILIP R. HALVERSON,  to me known,  who, being by me duly sworn, did
depose and say that they  respectively  reside at 2609 East 5th Street,  Duluth,
Minnesota 55812, and 3364 West Tischer Road, Duluth,  Minnesota 55803; that they
are respectively the Senior Vice President - Finance and Chief Financial Officer
and the Vice President, General Counsel and Secretary of MINNESOTA POWER & LIGHT
COMPANY,  one of the  corporations  described  in and which  executed  the above
instrument;  that they know the seal of said corporation;  that the seal affixed
to said  instrument is such corporate  seal;  that it was so affixed by order of
the Board of  Directors  of said  corporation,  and that they signed their names
thereto by like order.

     GIVEN under my hand and notarial seal this 20th day of November, 1997.



                                             Jeannette A. Atkinson
                                             -----------------------------------
                                                     JEANNETTE A. ATKINSON
                                             [SEAL]  NOTARY PUBLIC-MINNESOTA
                                                       ST. LOUIS COUNTY
                                             My Commission Expires Jan. 31, 2000


                                      -14-


STATE OF NEW YORK       )
                        )  SS:
COUNTY OF NEW YORK      )

     On this 20th day of November,  1997,  before me, a Notary Public within and
for said County,  personally appeared THOMAS B. ZAKRZEWSKI and REMO J. REALE, to
me personally  known,  who,  being each by me duly sworn,  did say that they are
respectively  an Assistant Vice President and an Assistant Vice President of THE
BANK OF NEW  YORK  of the  State  of New  York,  the  corporation  named  in the
foregoing  instrument;  that the seal affixed to the foregoing instrument is the
corporate seal of said  corporation;  that said instrument was signed and sealed
in behalf of said  corporation by authority of its Board of Directors;  and said
THOMAS B.  ZAKRZEWSKI and REMO J. REALE  acknowledged  said instrument to be the
free act and deed of said corporation.

     Personally  came before me on this 20th day of  November,  1997,  THOMAS B.
ZAKRZEWSKI,  to me known to be an Assistant Vice  President,  and REMO J. REALE,
known to me to be an Assistant  Vice  President,  of the above named THE BANK OF
NEW  YORK,  the  corporation  described  in and  which  executed  the  foregoing
instrument,  and to me  personally  known to be the persons who as such officers
executed the foregoing  instrument  in the name and behalf of said  corporation,
who,  being by me duly sworn did depose  and say and  acknowledge  that they are
respectively an Assistant Vice President and an Assistant Vice President of said
corporation;  that the seal affixed to said  instrument is the corporate seal of
said corporation;  and that they signed, sealed and delivered said instrument in
the  name  and on  behalf  of said  corporation  by  authority  of its  Board of
Directors,  and said  THOMAS  B.  ZAKRZEWSKI  and REMO J.  REALE  then and there
acknowledged said instrument to be the free act and deed of said corporation and
that such corporation executed the same.

     On the 20th day of  November,  1997,  before me  personally  came THOMAS B.
ZAKRZEWSKI  and REMO J. REALE,  to me known,  who,  being by me duly sworn,  did
depose and say that they respectively reside at 63 Sargent Road,  Freehold,  New
Jersey  07728 and 111  Jackson  Street,  Garden  City,  New York;  that they are
respectively  an Assistant Vice President and an Assistant Vice President of THE
BANK OF NEW YORK,  one of the  corporations  described in and which executed the
above  instrument;  that they know the seal of said  corporation;  that the seal
affixed to said  instrument is such  corporate  seal;  that it was so affixed by
order of the Board of Directors of said corporation,  and that they signed their
names thereto by like order.

     GIVEN under my hand and notarial seal this 20th day of November, 1997.


    [SEAL                                   William J. Cassels
 William J. Cassels                         ------------------------------------
     Notary                                 William J. Cassels
     Public                                 Notary Public, State of New York
 State of New York]                         No. 01CA5027729
                                            Qualified in Bronx County
                                            Certificate Filed in New York County
                                            Commission Expires May 16, 1998




                                      -15-


STATE OF NEW YORK       )
                        )  SS:
COUNTY OF NEW YORK      )


     On this 20th day of  November,  1997  before me  personally  appeared W. T.
CUNNINGHAM,  to me known to be the  person  described  in and who  executed  the
foregoing instrument, and acknowledged that he executed the same as his free act
and deed.

     Personally came before me this 20th day of November,  1997, the above named
W. T.  CUNNINGHAM,  to me known to be the  person  who  executed  the  foregoing
instrument, and acknowledged the same.

     On the  20th  day of  November,  1997,  before  me  personally  came  W. T.
CUNNINGHAM,  to me known to be the  person  described  in and who  executed  the
foregoing instrument, and acknowledged that he executed the same.

     GIVEN under my hand and notarial seal this 20th day of November, 1997.


      [SEAL                                 William J. Cassels
   William J. Cassels                       ------------------------------------
        Notary                              William J. Cassels
        Public                              Notary Public, State of New York
  State of New York]                        No. 01CA5027729
                                            Qualified in Bronx County
                                            Certificate Filed in New York County
                                            Commission Expires May 16, 1998