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                                                                       EXHIBIT 5


                         [HOLLAND & HART LLP LETTERHEAD]



                               September 17, 2001




First Interstate BancSystem, Inc.
401 North 31st Street
Billings, Montana  59101


Ladies and Gentlemen:

         We have acted as counsel to First Interstate BancSystem, Inc., a
Montana corporation (the "Company"), in connection with the registration under
the Securities Act of 1933 (the "Act") of 750,000 shares of the Company's common
stock, no par value (the "Shares"), to be offered upon the terms and subject to
the conditions set forth in the First Interstate BancSystem, Inc. 2001 Stock
Option Plan (the "Plan").

         In connection therewith, we have examined originals or copies,
certified or otherwise identified to our satisfaction, of the Articles of
Incorporation of the Company, the Bylaws of the Company, the Plan, records of
relevant corporate proceedings with respect to the offering of the Shares and
such other documents, instruments and corporate proceedings with respect to the
offering of the Shares as we have deemed necessary or appropriate. We have also
reviewed the Company's Registration Statement on Form S-8 (the "Registration
Statement") to be filed under the Act with the Securities and Exchange
Commission on September 17, 2001 with respect to the Shares.

         We have assumed the authenticity and completeness of all records,
certificates and other instruments submitted to us as originals, the conformity
to original documents of all records, certificates and other instruments
submitted to us as copies, the authenticity and completeness of the originals of
those records, certificates and other instruments submitted to us as copies and
the correctness of all statements of fact contained in all records, certificates
and other instruments that we have examined.


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First Interstate BancSystem, Inc.
September 17, 2001
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         Based on the foregoing, we are of the opinion that the Shares have been
duly authorized and, when issued in accordance with the terms of the Plan, will
be validly issued, fully paid and non-assessable.

         The opinion expressed herein is based solely upon and is limited to the
general corporation laws of the State of Montana and the federal laws of the
United States of America, to the extent applicable.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.


                                       Very truly yours,


                                       /s/ Holland & Hart LLP