EXHIBIT 5.1 [LETTERHEAD OF PILLSBURY MADISON & SUTRO] September 8, 1994 Westamerica Bancorporation 1108 Fifth Avenue San Rafael, California 94901 Ladies and Gentlemen: With reference to the Registration Statement on Form S-4 filed by Westamerica Bancorporation, a California corporation ("WABC"), with the Securities and Exchange Commission in connection with the registration under the Securities Act of 1933, as amended, of 1,400,000 shares of WABC Common Stock, no par value (the "Shares"), to be issued in connection with the merger contemplated by the Agreement and Plan of Reorganization dated as of July 25, 1994, as amended (the "Agreement"), among WABC, PV Financial, a California corporation, and Pacific Valley National Bank, a national banking association, and a related Agreement of Merger (the "Merger Agreement," and collectively with the Agreement, the "Agreements") which Agreements are described therein and filed as an exhibit thereto: We are of the opinion that the Shares have been duly authorized and, when issued in accordance with the Agreements, will be legally issued, fully paid and nonassessable. We hereby consent to the filing of this opinion as Exhibit 5.1 to the Registration Statement and to the use of our name under the caption "Legal Matters" in the Registration Statement and in the Proxy Statement/Prospectus included therein. Very truly yours, /s/ PILLSBURY MADISON & SUTRO