Exhibit 5(b)


                         LEBOEUF, LAMB, GREENE & MACRAE
                                     L.L.P.
       A LIMITED LIABILITY PARTNERSHIP INCLUDING PROFESSIONAL CORPORATIONS
                              125 WEST 55TH STREET
                             NEW YORK, NY 10019-5389


                                                March 23, 2000


MDU Resources Group, Inc.
Schuchart Building
918 East Divide Avenue
P.O. Box 5650
Bismarck, North Dakota  58506-5650

Ladies and Gentlemen:

     We are  acting  as  counsel  for MDU  Resources  Group,  Inc.,  a  Delaware
corporation (the "Company"),  in connection with the preparation and filing with
the Securities and Exchange  Commission (the "Commission")  under the Securities
Act of 1933, as amended (the  "Securities  Act"), of the Company's  registration
statement  on  Form  S-8  (the   "Registration   Statement")   relating  to  the
registration  of 420,000 shares of the Company's  common stock,  par value $1.00
per share (the "Stock"),  and the  preference  share  purchase  rights  attached
thereto (the "Rights"), to be issued pursuant to the Company's 1998 Option Award
Program (the "Plan").

     In  connection  with this opinion,  we have  examined  originals or copies,
certified or otherwise  identified to our satisfaction,  of (i) the Registration
Statement, (ii) the Rights Agreement, dated as of November 12, 1998, between the
Company and Norwest Bank,  Minnesota,  N.A., as Rights Agent,  together with the
exhibits  thereto  (the "Rights  Agreement"),  pursuant to which the Rights were
created,  (iii)  resolutions  of the Board of Directors  of the  Company,  dated
August 12, 1999,  authorizing the issuance of the Stock,  (iv) the orders of the
Federal Energy Regulatory  Commission dated February 9, 2000, the Montana Public
Service  Commission dated January 11, 2000 and the Public Service  Commission of
Wyoming dated January 18, 2000,  and (v) such other  instruments,  certificates,
records and documents,  and such matters of law, as we have considered necessary
or appropriate for the purposes hereof. In such examination, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as  originals,  the  conformity  to the  original  documents  of  all  documents
submitted to us as copies and the  authenticity  of the originals of such latter
documents. As to any facts material to our opinion, we have, when relevant facts
were not  independently  established,  relied  upon the  aforesaid  Registration
Statement, Rights



Agreement,   resolutions,   orders,  instruments,   certificates,   records  and
documents. We have also assumed the regularity of all corporate procedures.

     Based upon the foregoing, and subject to the limitations and qualifications
contained in this opinion, we are of the opinion that:

     1.   When

          (a)  appropriate  authorizations  by  the  Federal  Energy  Regulatory
               Commission,  the Montana Public Service Commission and the Public
               Service  Commission  of Wyoming  with respect to the issuance and
               sale of the Stock shall have been granted,

          (b)  the Company's Board of Directors or a duly  authorized  committee
               thereof shall have approved the issuance and sale of the Stock by
               the Company,

          (c)  the Registration  Statement shall have become effective under the
               Securities Act and

          (d)  the Stock  shall  have  been  issued,  delivered  and paid for in
               accordance  with the  terms of the  Plan  and the  corporate  and
               governmental  authorizations  and instruments  referred to above,
               the Stock will be validly issued, fully paid and non-assessable.

     2. The  Rights,  when  issued in  accordance  with the terms of the  Rights
Agreement, will be validly issued.

     We  express  no opinion  with  respect  to any laws other than the  General
Corporation  Law of the State of  Delaware,  the  applicable  provisions  of the
Delaware  Constitution  and the reported  judicial  decisions  interpreting  the
Delaware  laws,  and the  federal  laws of the  United  States.  Insofar as this
opinion involves matters of the laws of the States of Montana,  North Dakota and
Wyoming,  we have relied upon the opinion of even date herewith addressed to you
by Lester H. Loble,  II, Vice  President,  General  Counsel and Secretary of the
Company.

     We  hereby  consent  to the  use  of  this  opinion  as an  exhibit  to the
Registration  Statement.  In giving such consent, we do not thereby concede that
we are within the category of persons whose consent is required  under Section 7
of the Securities Act or the rules and regulations of the Commission thereunder.

                                    Very truly yours,


                                    /s/ LeBoeuf, Lamb, Greene & MacRae, L.L.P.