Based on previous papers and results from recent investigations, we present the biological and ecological characteristics of Japanese strains of Gymnodinium catenatum. Distribution, oceanographic conditions for blooming and the toxin composition of this toxic dinoflagellate in Japan including cysts in the sediment will be detailed. Recently, G. catenatum blooms have been increasing in the coastal waters of western Japan, e.g. Kyusyu and western Shikoku. During the bloom periods, wild oyster, short-necked clam and cultured noble scallop have been contaminated by PSP. Before 1996, the PSP events by G. catenatum had been restricted to Senzaki Bay, Japan Sea coast of western Honshu, where PSP by this species was first confirmed in 1986. Throughout the study sites in the coastal waters of western Japan, G. catenatum blooms have been observed in enclosed bay areas over a wide range of water temperatures, 6-27 degrees centigrade, and under different seasons. Major toxin components of C1+C2, GTX6 and GTX5 in the strains we examined are the same in other Japanese strains so far examined. Cysts in sediments were found throughout the study sites, although the cyst densities were low levels with below 10 cysts/g of wet sediment. Therefore, expansion of G. catenatum distribution in western Japanese waters shall be expected with fears of further PSP outbreaks.