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                                                                     Exhibit 5.1


                     [BROBECK PHLEGER & HARRISON LETTERHEAD]



                                October 21, 1999


Corsair Communications, Inc.
3408 Hillview Avenue
Palo Alto, California  94304

        Re:    Corsair Communications, Inc. - Registration Statement for
               Offering of 1,150,000 Shares of Common Stock

Ladies and Gentlemen:

        We have acted as counsel to Corsair Communications, Inc., a Delaware
corporation (the "Company"), in connection with the registration on Form S-8
(the "Registration Statement") under the Securities Act of 1933, as amended, of
750,000 shares of common stock under the Company's 1997 Stock Incentive Plan
(the "Incentive Plan") and 400,000 shares of common stock under the Company's
1997 Employee Stock Purchase Plan (the "Purchase Plan") (collectively, the
"Shares").

        This opinion is being furnished in accordance with the requirements of
Item 8 of Form S-8 and Item 601(b)(5)(i) of Regulation S-K.

        We have reviewed the Company's charter documents and the corporate
proceedings taken by the Company in connection with the establishment and
amendment of the Incentive Plan and the Purchase Plan. Based on such review, we
are of the opinion that if, as and when the Shares are issued and sold (and the
consideration therefor received) in accordance with the Incentive Plan and the
Purchase Plan (including any stock option agreements or stock issuance
agreements or stock purchase agreements thereunder) and in accordance with the
Registration Statement, such Shares will be duly authorized, legally issued,
fully paid and nonassessable.

        We consent to the filing of this opinion letter as Exhibit 5.1 to the
Registration Statement.

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Corsair Communications, Inc.                                    October 21, 1999
                                                                          Page 2



        This opinion letter is rendered as of the date first written above and
we disclaim any obligation to advise you of facts, circumstances, events or
developments which hereafter may be brought to our attention and which may
alter, affect or modify the opinion expressed herein. Our opinion is expressly
limited to the matters set forth above and we render no opinion, whether by
implication or otherwise, as to any other matters relating to the Company, the
Incentive Plan or the Purchase Plan, or the Shares issuable under the Incentive
Plan or the Purchase Plan.

                                        Very truly yours,

                                        /s/ BROBECK PHLEGER & HARRISON LLP

                                        BROBECK PHLEGER & HARRISON LLP