Large amounts of Microcystis aeruginosa were harvested from several lakes, ponds, reservoirs and moats in Tokyo and adjacent regions, and the toxicity of its water extract was examined by intraperitoneal injection in mice. Toxicity expressed as death of mice was found in the samples from five out of ten water bodies. The effects of temperature and proteases on toxicity of M. aeruginosa were also investigated in the samples from Lake Suwa. It was suggested that the toxin consisted of heat stable peptide or protein.