The occurrence of Chattonella antiqua red tide was observed in northern Hiroshima Bay during the late summer of 1990. Measurements of seasonal changes in the densities of Chattonella cysts in surface sediments (top 1 cm depth) at two stations were made, and the results indicate that abundant cyst depositions (600-700 cysts/cm2) occurred during the red tide period. The small cells of Chattonella before cyst formation were observed in water columns, and the concentrations of the small cells were higher at the bottom layer than at the surface, confirming the previous suggestion of descending characteristics of cyst-forming small cells. The bottom water temperature reached 20°C, which is a level for active germination of Chattonella cysts, in late July in the summer of 1990. Integrated irradiation time during the period from July 21 to August 31 (42 days) 1990 was rather longer than in normal years, suggesting that light conditions for the vegetative cells just after the germination of cysts are significant factors for developing Chattonella red tide in northern Hiroshima Bay.