Bull. Jpn. Soc. Fish. Oceanogr. 73(2), Page 57-66, 2009   Distribution and abundance of resting cysts of the toxic dinoflagellates Alexandrium tamarense and A. catenella in 2006 and 2007 in Osaka Bay Keigo Yamamoto1†, Yasunobu Nabeshima1, Mineo Yamaguchi2 and Shigeru Itakura2* 1 Marine Fisheries Research Center, Research Institute of Environment, Agriculture and Fisheries, Osaka Prefectural Government, 2926-1 Tanigawa, Tanagawa, Misaki, Sennan, Osaka 599-0311, Japan 2 National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, 2-17-5 Maruishi, Hatsukaichi, Hiroshima 739-0452, Japan * Research and Technological Guidance Division, Resources Enhancement Promotion Department, Fisheries Agency, 1-2-1 Kasumigaseki, Chiyoda-ku, Tokyo 100-8907, Japan † YamamotoK@o-suishi.zaqrs.jp Distribution and abundance of resting cysts of Alexandrium tamarense and A. catenella were invetigated in the surface of bottom sediment of Osaka Bay in October, 2006 and in May and October 2007 using a primuline-staining direct count method. Cysts of Alexandrium spp. were found at all stations examined. In 2006, the cyst density ranged from 2 to 112 cysts⋅m−3 and the cysts were abundant at north-east coast of Osaka Bay and the intermediate area between Akashi Strait and Kitan Strait. In 2007, higher densities were observed after an occurrence of A. tamarense red tide, and the cyst density varied from 291 to 5,683 cysts⋅m−3 and 190 to 3,275 cysts⋅m−3 in May and October, respectively. These facts show an increasing tendency of cyst abundance from the previous research in 1993. The cysts were most abundant at the costal waters off Izumiotsu City and Kishiwada City in the middle-east coast of the bay and the high densities were also observed at fishing ports. These areas agreed with the area of dense bloom in 2007, and have a potential for becoming seed banks of future blooms. These results suggest that the risk of Alexandrium bloom and shellfish poisoning in Osaka Bay has increased and careful monitoring are necessary to prevent paralytic shellfish poisoning (PSP) incidents in the bay. Key words: toxic dinoflagellate, Alexandrium, resting cyst, distribution, Osaka Bay, paralytic shellfish poisoning (PSP)