In spring of 1996, three species of bivalves, scallop Chlamys nobilis, mussel Mytilus edulis, and short-necked clam Tapes japonica, were toxified in Kamae, Oita Prefecture, Japan. In this bay, the cultured scallop C. nobilis has been infested with the dinoflagellate Alexandrium catenella almost every year, but the plankton was not seen this year. Instead, at the beginning of April 1996, a toxic dinoflagellate, which was later identified as Gymnodinium catenatum, appeared (maximal density: 44, 350 cells/L). The toxicity score of C. nobilis specimens reached the maximum (81MU/g, digestive gland) in early May, while M. edulis and T. japonica showed the maximum scores of 253MU/g and 28MU/g at the end of April, respectively. The toxins from the three species of bivalves closely resembled each other, consisting of C1 (Protogonyautoxin 1, PX1), C2 (PX2) and Gonyautoxin 6 (GTX6) as the major components. The toxin profile of G. catenatum was similar to that of the bivalves. This result indicates that G. catenatum may be a causative dinoflagellate for the PSP infestation of bivalves in Kamae.