Fifty species (42 genera and 29 families) of marine epibiotic invertebrates were found on thomy oysters, Spondylus princeps unicolor, from Puerto Escondido Bay, eastem Baja California Sur, México. These included polychaetes (31 species), molluscs (8), crustaceans (4), ophiuroids (3), and sponges, sipunculids, nemerteans and hydroids (one species each). The taxa were tropical and subtropical. The biogeographic component were: Pacific (46%), widely distributed (24%), endemic (14%), amphiamerican (12%) and disjunct (4%).