Pododesmus cepio Gray (1850), a bivalve mollusk of Family Anomiidae, is native to the Pacific coast of North America and common in Tomales Bay, California. A small population of the European oyster, Ostrea edulis Linnaeus (1758), is being raised experimentally at the Tomales Bay Oyster Company. The histological study of the seasonal gonadal changes of these two species, in Tomales Bay, is described below. The study of gametogenesis in O. edulis in Tomales Bay is desired to evaluate the reproductive adaptation of this animal to a new set of ecological conditions. The inclusion of the native P. cepio in this study allows comparison of the gonadal development of these two species. Such a comparison aids not only in understanding the seasonal gonadal changes in both species, but also in examining the adjustment of the introduced form, O. edulis, to a new environment.