Abundance and diversity of the estuarine fish of three mangrove areas in the Gulf of Nicoya, Pacific coast of Costa Rica, were determined between April 1992 and June 1993. Seventy four species were found (25 families, 44 genera). The total number of individuals captured was 1 679 and the total biomass was 320.581 Kg. Arius seemanni accounted for 12.0% of the total number of individual s and contributed most of the total biomass (28.5%). The family Ariidae comprised 31.5% of the total number of individuals. 57.1 % of total biomass and 16.0% of total number of species. The main community component in the three sites was an occasional visitor, comprising 78.4%, 65.3% and 76.3% of species. The diversity index H' fluctuated between 2.39 and 3.18 and the biomass index between 2.08 and 2.86. The moderately low changes in biological index values, together with high annual H' values indicate that these mangrove fish populations are not adversely stressed by environmental contamination in the gulf' s shore