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Exhibit 31.1

CERTIFICATION UNDER
SECTION 302 OF
THE SARBANES OXLEY ACT OF 2002


I, David L. Payne, Chief Executive Officer of the Company, certify that:

1. I have reviewed this quarterly report for the period ending June
30, 2005 on Form 10-Q of Westamerica Bancorporation;

2. Based on my knowledge, this quarterly report does not contain any
untrue statement of a material fact or omit to state a material fact
necessary to make the statements made, in light of the circumstances
under which such statements were made, not misleading with respect
to the period covered by this report;

3. Based on my knowledge, the financial statements, and other
financial information included in this report, fairly present in all
material respects the financial condition, results of operations and
cash flows of the registrant as of, and for, the periods presented
in this report;

4. The registrant's other certifying officer(s) and I are
responsible for establishing and maintaining disclosure controls and
procedures (as defined in Exchange Act Rules 13a-15(e) and
15d-15(e)) and internal control over financial reporting (as defined
in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the registrant
and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure
controls and procedures to be designed under our supervision, to ensure that
material information relating to the registrant, including its consolidated
subsidiaries, is made known to us by others within those entities,
particularly during the period in which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such
internal control over financial reporting to be designed under our
supervision, to provide reasonable assurance regarding the reliability of
financial reporting and the preparation of financial statements for external
purposes in accordance with generally accepted accounting principles;

(c) Evaluated the effectiveness of the registrant's disclosure controls and
procedures and presented in this report our conclusions about the
effectiveness of the disclosure controls and procedures, as of the end of the
period covered by this report based on such evaluation; and

(d) Disclosed in this report any change in the registrant's internal control
over financial reporting that occurred during the registrant's most recent
fiscal quarter (the registrant's fourth fiscal quarter in the case of an
annual report) that has materially affected, or is reasonably likely to
materially affect, the registrant's internal control over financial reporting;
and

5. The registrant's other certifying officer(s) and I have
disclosed, based on our most recent evaluation of internal control
over financial reporting, to the registrant's auditors and the audit
committee of registrant's board of directors (or persons performing
the equivalent functions):

(a) All significant deficiencies and material weaknesses in the design or
operation of internal control over financial reporting which are reasonably
likely to adversely affect the registrant's ability to record, process,
summarize and report financial information; and

(b) Any fraud, whether or not material, that involves management or other
employees who have a significant role in the registrant's internal control
over financial reporting.


/s/  David L. Payne                                   August 9, 2005
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David L. Payne                                        Date
Chairman, President and Chief Executive Officer