Exhibit 11 WESTAMERICA BANCORPORATION Computation of Earnings Per Share on Common and Common Equivalent Shares and on Common Shares Assuming Full Dilution - ----------------------------------------------------------------------------- For the three For the nine months ended months ended September 30, September 30, (In thousands, except per share data 1998 1997 1998 1997 - ----------------------------------------------------------------------------- Weighted average number of common shares outstanding - basic 41,600 43,171 42,312 43,064 Add exercise of options reduced by the number of shares that could have been purchased with the proceeds of such exercise 665 837 736 765 - ----------------------------------------------------------------------------- Weighted average number of common shares outstanding - diluted 42,265 44,008 43,048 43,829 ============================================================================= Net income $18,436 $17,660 $54,634 $29,838 Basic earnings per share $0.44 $0.41 $1.29 $0.69 Diluted earnings per share $0.44 $0.40 $1.27 $0.68