Bull. Jpn. Soc. Fish. Oceanogr. 74(1), Page 13-19, 2010   Occurrence of the toxic benthic dinoflagellate Gambierdiscus toxicus in Ago Bay, central part of Japan Akira Ishikawa1† and Akira Kurashima1 1 Graduate School of Bioresources, Mie University, 1577 Kurima-machiya-cho, Tsu, Mie 514-8507, Japan † ishikawa@bio.mie-u.ac.jp Gambierdiscus toxicus is a benthic, toxin-producing armored dinoflagellate, often found on macroalgae and dead corals. This organism is the primary causative agent of ciguatera fish poisoning which occurs in tropical and subtropical regions. In the last decade, several incidents of ciguatera were reported even in the central parts of pacific coast of Japan, suggesting the possibility that G. toxicus has expanded its population to such temperate areas. In the present study, we investigated the abundance (density of the cells attached to macroalgae) of G. toxicus in November and December, 2008, at Hamajima in Ago Bay, at eastern part of Kii Peninsula. Gambierdiscus toxicus in Ago Bay did not exhibit a tendency of preference to attach to specific algae but showed a patchy distribution. Cell density on macroalgae ranged from 0-4.7 cells g-1 (of macroalga wet weight) (mean; 2.1 cells g-1) and from 0-2.5 cells g-1 (mean; 0.6 cells g-1) in November and December, respectively. Maximum density, 4.7 cells g-1, was recorded in the brown alga Padina arborescens (Phaeophyceae) in November. Considering that G. toxicus is a tropical and subtropical species, the population of this organism in Ago Bay possibly becomes larger in warmer seasons. The monitoring of the organism is thus necessary to predict and prevent ciguatera poisoning in temperate areas in Japan. Key words: Gambierdiscus toxicus, toxic benthic dinoflagellate, ciguatera, macroalgae, Hamajima, Ago Bay