The blooming of Microcystis aeruginosa Kotz. In the reservoir of Mae Kuang Udomtara dam, Chiang Mai, Thailand were investigated for 18 months during August 1996 - January 1998. The water quality in the reservoir classified by torphic level was found to be mesotrophic to eutrophic. The main problem of water quality in the reservoir was the proliferation of phytoplankton, Microcystis aeruginosa Kotz. Which secreted microcystin (hepatotoxin). It was found throughout the investigation in large amount during July 1996-January 1998. The factors effecting the proliferation were the amount of soluble reactive phosphorus and the total phosphours which showed negative correlation with the volume of water in the reservoir. The vertical changes of the physical, chemical and biological properties of water were more obvious especially the total biovolume and that of M. aeruginosa at the water surface and decreased at the lower levels. Six types of microcystin were isolated from the sample, microcystin RR was present in the highest concentration in this investigation followed by microcystin LR, Z- RR, YR, Z-LR and ThtyrR respectively. Toxicity tests with animals showed an LD50 for microcystin RR of 700-800 ng.kg^-1 body weitht, whilst the value for microcystin LR was 70 mg.kg^-1 body weight.