Polychlorinated biphenyls (as sum of 7 congeners), organochlorine pesticides and mercury contents were compared in ripe bivalve female ( 4.5 and 5.5 cm length) Anadara tuberculosa collected from Punta Morales, Puerto Alto and Jicaral mangroves areas, located in the Nicoya Gulf on the Pacific coast of Costa Rica ( 10°N.85°W). The total PCB content in the bivalves is significantly correlated (p<.01) with their lipid content. This is not the case for the total mercury content. Therefore, PCB levels in different regions and species were compared on a lipid weight basis but the total mercury levels comparison were done on a dry weight basis. PCBs, organochlorine pesticides and mercury were also compared between male and female bivalves collected from the Jicaral mangrove. Punta Morales is the place with the highest (p <.01) PCB level by lipid weight and Jicaral the one with the lowest (p <.01 ). By dry weight the differences are also significant (p <.01) between Puerto Alto and Punta Morales but not between Punta Morales and Jicaral. In a lipid base males have significantly (p <.01) higher concentrations than the females but on a dry weight base the concentrations measured in females are higher. This difference is, however, not significant. The total mercury concentration (by dry weight) found was also higher in the molluscs coming from Punta Morales followed by the ones of Puerto Alto and Jicaral. These noted differences are not highly significant (p<.03) The total mercury content in males and females is similar. On a lipid weight basis the aldrin was not different between the three places. The highest concentrations of lindane was measured in Puerto Alto (p<.05). The heptachlor was higher (p<.05) in Punta Morales than in Jicaral. From the three pesticides, heptachlor was the one which had more concentration in the females than in the males (p<.05). Polychlorinated biphenyls, organochlorine and mercury contents for different species collected in Punta Morales estuary and for sediments collected in the three places are also given.