The study of water quality and phytoplankton in the reservoir of Mae Kuand Udomtara Dam, Chiang Mai, were investigated for 18 months during August 1996 - January 1998. The water quality in the reservoir classified by standard surface water quality was found to be the third category and relatively clean for household consumption when property treated. However, by the trophic levels, the reservoir was mestrophic to eutrophic. The main problem of water quality in the reservoir was the proliferation of phytoplankton, Microcystis aeruginosa Kutz. which secretd 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 phosphorus 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 dedreased at the lower levels. Morever, the total biovolume correlated with the primary productively and chlorophyll a. One hundred and twenty two species of phytoplankton were found. The greatest number of species was in Chlorophyceae (35%) followed by Zygnemaphyceae (20%), Diatomophyceae (14%), Cyanophyceae (9%), Euglenophyceae (9%), Cryptophyceae(6%), Dinophyceae(5%), Xanthophyceae(2%) respectively. The large proportions were cosmopolitan, the minority were tropical or warm temperate and tropical species. The dominant species was M. aeruginosa which could be used to indicate the eutrophic quality.