The Microcystis waterblooms were collected from the eutrophic Chao Lake from Sept.to Nov. in 1986. Only the waterblooms of M. aeruginosa from Tang-shi on Sept.8th and M.flos-aquae from Zhong-miao on Sept.9th were found toxic to mice after intraperitoneal injection bioassay. These two samples contained fast death factor (FDF) which produced by the algal cells. The minimum lethal dose (LDmin) of toxic M.aeruginosa waterbloom was 90mg/Kg of mice, and M.flos-aquae waterbloom, about 1000 mg/Kg of mice. When the injected dosage larger than the LDmin, the animals would die in 4 hours. The symptoms were identical to that of other heptotoxin-producing Microcystis blooms and strains reported from other parts of the world. The toxin of Chao Lake M.aeruginosa waterbloom was heatstable peptide, and was extracted by methanol/butanol. separated by DEAE-SephadexA 50 ion-exchange chromatography, and purified by high performance liquid chromatography (HPLC) The total toxicity recovery was about 16.36 percent, an the purity of the purified to xin was about 95.75 percent measured by HPLC. The content of toxin in M.aeruginosa cells was about 4.45 mg 7820 toxin/g dry weight alga, and the LDmin of the purified toxin was about 0.33mg/Kg of mice. On chloride acid hydrolysis the amino acid composition of the toxin was Asp(1),Clu(3), Ala(3), Leu(3), Arg(2), and was similar to that of Cyanoginosin-LR.