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                                                                    EXHIBIT 5.01

                        OPINION OF FENWICK AND WEST LLP




                               February 25, 1998

HNC Software Inc.
5930 Cornerstone Court West
San Diego, California 92121-3728

Gentlemen/Ladies:

          At your request, we have examined the Registration Statement on Form
S-8 (the "Registration Statement") to be filed by you with the Securities and
Exchange Commission (the "Commission") on or about February 25, 1998 in
connection with the registration under the Securities Act of 1933, as amended,
of 1,000,000 shares of the Common Stock, $0.001 par value (the "Common Stock")
of HNC Software Inc, a Delaware corporation (the "Company"), subject to issuance
by the Company under its 1998 Stock Option Plan (the "HNC 1998 Plan").

         In rendering this opinion, we have examined the following:

         (1)      your registration statement on Form S-1 (File Number 33-91932)
                  filed with and declared effective by the Commission on June
                  20, 1995, together with the Exhibits filed as a part thereof;

         (2)      your registration statement on Form 8-A filed with the
                  Commission on May 26, 1995, together with the order of
                  effectiveness issued by the Commission therefor on June 20,
                  1995;

         (3)      the Registration Statement, together with the Exhibits filed
                  as a part thereof;

         (4)      the HNC 1998 Plan and related stock option grant and exercise
                  agreement forms;

         (5)      the Prospectus prepared in connection with the Registration
                  Statement;

         (6)      the Restated Certificate of Incorporation of the Company filed
                  with the Delaware Secretary of State on June 13, 1996 and the
                  Bylaws of the Company, both as filed by the Company with its
                  Report on Form 10-Q for the quarter ended June 30, 1996;

         (7)      the minutes of meetings and actions by written consent of the
                  stockholders and Board of Directors of the Company that are
                  contained in your minute books that are in our possession;

         (8)      the stock records for the Company that you have provided to us
                  (consisting of a list of stockholders issued by your transfer
                  agent, Boston EquiServe LLP and a list of all holders of
                  outstanding options and warrants to purchase the Company's
                  capital stock that was prepared by you and dated February 20, 
                  1998); and

         (9)      a Management Certificate addressed to us and dated of even
                  date herewith executed by the Company containing certain
                  factual and other representations.

         In our examination of documents for purposes of this opinion, we have
assumed, and express no opinion as to, the genuineness of all signatures on
original documents, the authenticity of all documents submitted to us as
originals, the conformity to originals of all documents submitted to us as
copies, the legal capacity of all natural persons executing the same, the lack
of any undisclosed terminations,



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modifications, waivers or amendments to any documents reviewed by us and the due
execution and delivery of all documents where due execution and delivery are
prerequisites to the effectiveness thereof.

         As to matters of fact relevant to this opinion, we have relied solely
upon our examination of the documents referred to above and have assumed the
current accuracy and completeness of the information obtained from records
included in the documents referred to above. We have made no independent
investigation or other attempt to verify the accuracy of any of such information
or to determine the existence or non-existence of any other factual matters;
however, we are not aware of any facts that would lead us to believe that the
opinion expressed herein is not accurate.

         Our opinion below is given on the assumption that the 1,000,000 shares
of Common Stock of the Company referred to in such paragraph may not be issued
and sold by the Company in accordance with the HNC 1998 Plan unless and until
such shares, at the time in question, are (i) explicitly reserved and available
for issuance under the HNC 1998 Plan or (ii) become issuable under the HNC 1998
Plan in the future by virtue of the terms of Section 2.1 of the HNC 1998 Plan,
which provide that certain shares issuable upon exercise of stock options
granted under the HNC 1998 Plan that expire or become unexercisable without
having been exercised become available again for grant and issuance under the
HNC 1998 Plan.

         Based upon the foregoing, it is our opinion that:

         The 1,000,000 shares of Common Stock that may be issued and sold by you
upon the exercise of stock options to be awarded under the HNC 1998 Plan, when
issued and sold in accordance with the HNC 1998 Plan and the stock option
agreements to be entered into thereunder, and in the manner referred to in the
Prospectus associated with the HNC 1998 Plan and the Registration Statement,
will be validly issued, fully paid and nonassessable.

         We consent to the use of this opinion as an exhibit to the Registration
Statement and further consent to all references to us, if any, in the
Registration Statement, the Prospectus constituting a part thereof and any
amendments thereto.

         This opinion speaks only as of its date and is intended solely for your
use as an exhibit to the Registration Statement for the purpose of the above
issuance of securities referred to in the above opinion and is not to be relied
upon for any other purpose.

                                Very truly yours,

                                FENWICK & WEST LLP