Indistinguishable prodissoconch and adult shells, ease of hybridization, and normal meiosis and mitosis in the hybrids support the contention that the Portuguese oyster Crassostrea angulata and the Japanese oyster C. gigas are the same species. By rules of priority the Portuguese is a subspecies of the Japanese oyster. Two hypotheses are given to explain the widely disjunct distribution, by both species arising from a common ancestor or by undocumented introduction. Both oysters are of considerable commercial importance and crossing the two should result in progeny that can be selected for superior commercial traits.