Reproduction of the shrimp Penaeus occidentalis was studied from June 26 through July 27, 1992 near Curazao beach, Isla Chira, Golfo de Nicoya, Costa Rica. This is the first documentation of reproduction oí a penaeid shrimp in Golfo de Nicoya. Spawning behavior was divided in five stages based on these observations, and sperm-egg interaction was evaluated with the in vitro spawning technique. Spawned eggs presented a cortical reaction during the first 30 min after being released, and sperm experienced primary binding to egg vitelline envelope, but an acrosome reaction was not activated. There appear to be differences in fertilization mechanisms between open and close thelycum penaeid shrimps. Wild P. occidentalis males presented heavy spermatophores compared 10 other open thelycum shrimps. Sperm count (49.52 million per compound spermatophore) and sperm abnormalities (22.0 %) were similar to those of wild P. setiferus from the Gulf of Mexico.
