November 20, 1996


Pegasystems Inc.
101 Main Street
Cambridge, MA  02142

Gentlemen:

        This opinion is delivered to you in connection with the registration
statement (the "Registration Statement") on Form S-1 of Pegasystems Inc. (the
"Company") filed on November 20, 1996 with the Securities and Exchange
Commission by the Company under the Securities Act of 1933, as amended (the
"Securities Act"), for registration under said Act of 2,581,750 shares of the
common stock, $.01 par value (the "Common Stock"), of the Company.

        We are familiar with the Restated Articles of Organization of the
Company, the corporate minute book and the Restated by-laws of the Company, and
the Registration Statement. We have also made such further investigation as we
have deemed necessary for the purposes of this opinion.

        Based upon and subject to the foregoing, we are of the opinion that the
shares of Common Stock to be sold by the Company pursuant to the prospectus
contained in the Registration Statement (the "Prospectus") have been validly
authorized for issuance and, when issued against receipt of the purchase price
described in the Prospectus, will be legally issued, fully paid and
nonassessable.



        We understand that this opinion is to be used in connection with the
Registration Statement. We consent to the filing of this opinion as an exhibit
to the Registration Statement, the incorporation by reference of this opinion in
any abbreviated registration statement in connection with the Common Stock
pursuant to Rule 462(b) promulgated under the Securities Act, and the reference
to our firm in the Prospectus under the caption "Legal Matters."

                                        Very truly yours,


                                        CHOATE, HALL & STEWART
DS1/305140