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                                                                    EXHIBIT 5.01


                                February 5, 1999

HNC Software Inc.
5930 Carnerstone Court West
San Diego, CA 92121-3728

Gentlemen/Ladies:

        At your request, we have examined the Registration Statement on Form S-8
(the "Registration Statement") filed by you with the Securities and Exchange
Commission (the "Commission") on or about February 5, 1999 in connection with
the registration under the Securities Act of 1933, as amended, of:

        (i)    an aggregate of 700,000 additional 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 1995 Equity Incentive Plan, as amended through
               November 20, 1998 (the "HNC Plan"); and

        (ii)   400,532 shares of the Company's Common Stock which are issuable
               upon the exercise of options originally granted by Aptex Software
               Inc., a formerly existing California corporation and subsidiary
               of the Company ("Aptex"), under the Aptex 1996 Equity Incentive
               Plan adopted effective September 5, 1996 (the "Aptex Plan") that
               have been assumed by the Company and converted into options to
               purchase shares of the Company's Common Stock (the "Assumed
               Options") pursuant to the Plan of Merger adopted by the Boards of
               Directors of both Aptex and the Company as of December 21, 1998
               (the "Merger 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 Prospectuses prepared in connection with the Registration
               Statement;

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        (5)    the Nasdaq National Market Listing of Additional Shares
               Notifications prepared in connection with the Registration
               Statement;

        (6)    the HNC Plan and related award grant and exercise agreement
               forms;

        (7)    the Aptex Plan and related award grant and exercise agreement
               forms;

        (8)    the Merger Plan;

        (9)    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;

        (10)   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;

        (11)   A certificate from your transfer agent Boston Equiserve dated of
               even date herewith, verifying the number of your issued and
               outstanding shares of capital stock as of the date hereof and a
               list of outstanding options to purchase shares of the Company's
               capital stock that was prepared by you and dated February 5, 1999
               verifying the number of such issued and outstanding securities;
               and

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

        We have also have confirmed your eligibility to use Form S-8 by
telephone call to the offices of the Commission.

        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 and the completeness of all documents
submitted to us as originals, the conformity to originals and completeness of
all documents submitted to us as copies, the legal capacity of all natural
persons executing the same, the lack of any undisclosed termination,
modification, waiver or amendment to any document reviewed by us and the due
authorization, execution and delivery of all documents where due authorization,
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 public
officials and records 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 cause us to believe that the
opinion expressed herein is not accurate.

        We are admitted to practice law in the State of California, and we
express no opinion herein with respect to the application or effect of the laws
of any jurisdiction other than the

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existing laws of the United States of America and the State of California and
(without reference to any case law or secondary sources) the existing Delaware
General Corporation Law.

        In connection with our opinion expressed below, we have assumed that, at
or prior to the time of the delivery of any shares of Stock, the Registration
Statement will have been declared effective under the Securities Act of 1933, as
amended, that the registration will apply to such shares of Stock and will not
have been modified or rescinded or been made subject to any Commission stop
order and that there will not have occurred any change in law affecting the
validity or enforceability of such shares of Stock.

        Our opinion in paragraph (i) below is given on the assumption that the
700,000 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 Plan unless and
until such shares, at the time in question, are (a) explicitly reserved and
available for issuance under the HNC Plan or (b) become issuable under the HNC
Plan in the future by virtue of the terms of Section 2.1 of the HNC Plan, which
provide that certain shares issuable upon exercise of stock options granted
under the Prior Plan (as the term is defined in the HNC Plan) that expire or
become unexercisable without having been exercised are available for grant and
issuance under the HNC Plan.

        Based upon the foregoing, it is our opinion that:

        (i)    the 700,000 additional shares of Common Stock that may be issued
               and sold by you upon the exercise of stock options, the purchase
               of restricted stock or awards of stock bonuses awarded or to be
               awarded under the HNC Plan, when issued and sold in accordance
               with the HNC Plan and the stock option, restricted stock purchase
               agreement or stock bonus agreements to be entered into
               thereunder, and in the manner referred to in the relevant
               Prospectus associated with the HNC Plan and the Registration
               Statement, will be validly issued, fully paid and nonassessable;
               and

        (ii)   The 400,532 shares of Common Stock that may be issued and sold by
               the Company pursuant to exercise of the Assumed Options, when
               issued and sold in the manner referred to in the Prospectus
               associated with the Assumed Options and the Registration
               Statement and in accordance with the Aptex Plan pursuant to which
               the Assumed Options were granted, 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.
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        This opinion speaks only as of its date and we assume no obligation to
update this opinion should circumstances change after the date hereof. This
opinion is intended solely for your use as an exhibit to the Registration
Statement for the purpose of the above sale of the Stock and is not to be relied
upon for any other purpose.

                                            Very truly yours,


                                            FENWICK & WEST LLP