Algicidal activity of a bacterium strain Cytophaga sp. AA8-2 against the harmful red tide causing alga Heterocapsa circularisquama was investigated. Physiological conditions of host cells, incubation temperature and existence of ambient organic substrates or coflexisting bacteria affected the lethal effect of Cytophaga sp. AA8-2. Bacterial lysis of H. circularisquama was caused more rapidly at higher incubation temperature (20–30°C). Growth of 6 among 7 H. circularisquama strains tested was inhibited by Cytophaga sp. AA8-2, the levels of which were varied. A part of H. circularisquama cells in a culture formed temporary cysts to survive the bacterial attack. The envelope of the temporary cyst of H. circularisquama was composed of a markedly thicker layered structure (209±72 nm) than that of the vegetative cell (40±15 nm).