[DORSEY & WHITNEY LLP OPINION LETTERHEAD]


                                                                     Exhibit 5.1

                                     June 17, 1997

First Bank System, Inc.
601 Second Avenue South
Minneapolis, Minnesota 55402-4302

      Re: Registration Statement on Form S-4

Ladies and Gentlemen:

            We have acted as counsel to First Bank System, Inc., a Delaware
corporation (the "Company"), in connection with a Registration Statement on Form
S-4 (the "Registration Statement") relating to shares of Common Stock, $1.25 par
value (the "Common Stock"), of New USBC (as hereinafter defined) and shares of 
8 1/8% Cumulative Preferred Stock, Series A, par value $1.00 (the "Preferred
Stock"), of New USBC to be issued in connection with the merger (the "Merger")
of U. S. Bancorp, an Oregon corporation, with and into the Company, as described
in the Joint Proxy Statement/Prospectus (the "Joint Proxy Statement/Prospectus")
constituting part of the Registration Statement. Following consummation of the
Merger, the Company will be renamed "U.S. Bancorp" ("New USBC").

            We have examined such documents 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 set forth below, 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, 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


First Bank System, Inc.
June 17, 1997
Page 2

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 of public officials.

            Based on the foregoing, we are of the opinion that the shares of the
Common Stock and the Preferred Stock to be issued in connection with the Merger,
when issued in accordance with the terms of the Merger Agreement (as defined in
the Joint Proxy Statement/Prospectus), will be duly authorized, validly issued,
fully paid and nonassessable.

            Our opinions expressed above are limited to the Delaware General
Corporation Law.

            We hereby consent to your filing of this opinion as an exhibit to
the Registration Statement, and to the reference to this firm under the heading
"Legal Opinions" in the Joint Proxy Statement/Prospectus.


                                               Very truly yours,


                                               Dorsey & Whitney LLP

PFC