Cysts of the red tide flagellate Heterosigma akashiwo (Raphidophyceae) were found for the first time from surface sediments collected in northern Hiroshima Bay in the Seto Inland Sea, Japan. The cysts were mostly spherical with a diameter of 8.0-12.0μm and were surrounded by mucilaginous material to which various particles such as sand grain, mud, and pieces of diatom frustule adhered. The cysts were generally solitary, but occasionally formed a cluster of several cysts. They were yellow-greenish to brownish in color. The cysts do not have clumps of black or dark brown materials as Chattonella cysts do. The cysts were generally smaller than vegetative cells. Live cysts had auto-fluorescing chloroplasts (4-11/cyst) under blue-light excitation. The nuclei of cysts (3.3-5.0μm in diameter) were smaller than those of vegetative cells. The germination occurred within a few days, mostly one or two days, when incubated in filtered-autoclaved seawater at 22°C with light.