EXHIBIT 5.1



                       [Dorsey & Whitney LLP letterhead]


The Board of Directors
First Bank System, Inc.
601 Second Avenue South
Minneapolis, Minnesota  55401

          Re:  First Bank System, Inc.
               Registration Statement on Form S-4
               File No. 333-16991
 
Ladies and Gentlemen:

          We have acted as counsel to First Bank System, Inc., a Delaware
corporation (the "Company") and Depositor of FBS Capital I, a Delaware business
trust (the "Trust"), in connection with a Registration Statement on Form S-4
(the "Registration Statement") relating to (i) the proposed issuance by the
Trust of $300,000,000 aggregate Liquidation Amount of the Trust's 8.09% Capital
Securities due November, 15, 2026 (the "New Capital Securities") registered
under the Securities Act of 1933, as amended (the "Securities Act"), in exchange
for up to $300,000,000 aggregate Liquidation Amount of the Trust's outstanding
8.09% Capital Securities due November 15, 2026 (the "Old Capital Securities");
(ii) the proposed issuance by the Company to the Trust of $309,279,000 aggregate
principal amount of the Company's 8.09% Junior Subordinated Debentures (the "New
Junior Subordinated Debentures") registered under the Securities Act, in
exchange for up to $309,279,000 aggregate principal of the Company's outstanding
8.09% Junior Subordinated Debentures (the "Old Junior Subordinated Debentures");
and (iii) the Company's guarantee (the "New Guarantee"), which guarantees the
payment of Distributions and payments on liquidation or redemption of the New
Capital Securities, registered under the Securities Act, in exchange for the
guarantee (the "Old Guarantee") which guarantees the payment of Distributions
and payments on liquidation or redemption of the Old Capital Securities. The New
Capital Securities are issuable under an Amended and Restated Trust Agreement
dated as of November 26, 1996 (the "Trust Agreement") between the Company, as
Depositor, Wilmington Trust Company, as Delaware Trustee and Property Trustee,
and the Administrative Agents named therein; the New Junior Subordinated
Debentures are issuable under an Indenture dated as of November 15, 1996 (the
"Indenture") between the Company and Wilmington Trust Company, as Debenture
Trustee; and the New Guarantee is issuable under the Guarantee Agreement dated
as of November 26, 1996 (the "Guarantee Agreement") between the Company and
Wilmington Trust Company, as Guarantee Trustee.

          We have examined such documents, including resolutions adopted by the
Board of Directors of the Company on November 16, 1996 (the "Resolutions"), and
have reviewed such questions of law as we have considered necessary and
appropriate for the purposes of our opinions set forth below. In rendering our
opinions, we have assumed the authenticity of all documents submitted to us as
originals, the genuineness of all signatures and the conformity to authentic
originals of all documents submitted to us as copies. We have also assumed the
legal capacity for all purposes relevant hereto of all natural persons and, with
respect to all parties to agreements or instruments relevant hereto other than
the Company and the Trust, that such parties had the requisite power and
authority (corporate or otherwise) to execute, deliver and perform such
agreements or instruments, that such agreements or instruments have been duly
authorized by all requisite action (corporate or otherwise), executed and
delivered by such parties and that such agreements or instruments are the valid,
binding and enforceable obligations of such parties. As to questions of fact
material to our opinions, we have relied upon certificates of officers of the
Company and the Trust and of public officials. Capitalized terms


The Board of Directors
First Bank System, Inc.
December 20, 1996
Page 2
 
used herein and not otherwise defined herein shall have the meanings assigned to
them in the Trust Agreement, the Indenture and the Guarantee Agreement, as
applicable.

          Based on the foregoing, we are of the opinion that:

          (1) The New Junior Subordinated Debentures have been duly authorized
by all requisite corporate action and, when executed and authenticated as
specified in the Indenture and delivered against surrender and cancellation of a
like amount of Old Junior Subordinated Debentures in the manner described in the
Registration Statement, the New Junior Subordinated Debentures will constitute
valid and binding obligations of the Company, enforceable in accordance with
their terms.

          (2) The New Guarantee has been duly authorized by all requisite
corporate action and, when executed as specified in the Guarantee Agreement and
delivered against surrender and cancellation of the Old Guarantee in the manner
described in the Registration Statement, the New Guarantee will constitute the
valid and binding obligation of the Company, enforceable in accordance with its
terms.

          The opinions set forth above are subject to the following
qualifications and exceptions:


          (a) Our opinions in paragraphs (1) and (2) above are subject to the
     effect of any applicable bankruptcy, insolvency, reorganization, moratorium
     or other similar law of general application affecting creditors' rights.

          (b) Our opinions in paragraphs (1) and (2) above are subject to the
     effect of general principles of equity, including (without limitation)
     concepts of materiality, reasonableness, good faith and fair dealing, and
     other similar doctrines affecting the enforceability of agreements
     generally (regardless of whether considered in a proceeding in equity or at
     law).

          (c) Minnesota Statutes (S) 290.371, Subd. 4, provides that any
     corporation required to file a Notice of Business Activities Report does
     not have a cause of action upon which it may bring suit under Minnesota law
     unless the corporation has filed a Notice of Business Activities Report and
     provides that the use of the courts of the State of Minnesota for all
     contracts 

The Board of Directors
First Bank System, Inc.
December 20, 1996
Page 3
 
     executed and all causes of action that arose before the end of any period
     for which a corporation failed to file a required report is precluded.
     Insofar as our opinion may relate to the valid, binding and enforceable
     character of any agreement under Minnesota law or in a Minnesota court, we
     have assumed that any party seeking to enforce such agreement has at all
     times been, and will continue at all times to be, exempt from the
     requirement of filing a Notice of Business Activities Report or, if not
     exempt, has duly filed, and will continue to duly file, all Notice of
     Business Activities Reports.


          Our opinions expressed above are limited to the laws of the States of
Minnesota and New York and the federal laws of the United States of America.

          We hereby consent to your filing of this opinion as an exhibit to the
Registration Statement, and to the reference to our firm under the caption
"Validity of the New Securities" contained in the Prospectus included
therein.

Dated:  December 20, 1996

                                    Very truly yours,

                                    /s/ Dorsey & Whitney LLP

                                    DORSEY & WHITNEY LLP

CFS