Along the Caribbean coast of Costa Rica, the populations of the indigenous oyster, Crassostrea rhizophorae, are very small, mainly due to the absence of mangroves. In search of cultivation sites for oysters, several studies on growth and survival were carried out at the Pacific coast. Also culture of imported C. gigas is attempted. Cultchless seed of C. rhizophorae, produced in the laboratory, and C. gigas imported from Chile were transplanted to different sites at the Pacific coast. Growth and survival were determined every month On a shrimp farm in Chomes (Puntarenas) survival of C. rhizophorae was only 6% after eight months. Growth was slow during this period (final shell height 56 mm, total weight 26 g). In ponds without shrimp Polydora caused high mortality and impeded growth. In Quepos (Puntarenas), within shrimp ponds, growth and survival of C. gigas were also bad, but in the water supply channel the Pacific oyster reached 80 mm shell height in nine months. Survival was > 50 %. In the Gulf of Nicoya (oceanic water, raft culture) both C. rhizophorae and C. gigas showed a fast initial growth during the first months, but high mortality caused by predators (fish species Balistidae and Diodontidae, and gastropods Cymatium and Thais spp.) obliged to stop the trials. Local fishermen collaborate to test methods to prevent predation and also competeion, mainly from barnacles and sponges.