This study of water quality in Ang Kaew Reservoir, Chiang Mai University using plankton as bioindicator was carried out during April 1996 - May 1998. It was found that four species of phytoplankton and two species of zooplankton can be used as bioindicator for the reservoir. Three species of phytoplankton i.e. Planktolyngbya limnetica Lemmerman, Aulacoseira granulata (Ehrenberg) Ralfs and Trachelomonas volvocina Ehrenberg indicated polluted water quality whilst Dinobryon divergens Imhof indicated clean water quality. Two species of zooplanktoni.e. Polyarthhra vulgaris indicated polluted water quality whilst Tetramastrix opoliensis indicated clean water quality. Water quality was assessed by the standard surface water quality of Thailand classification and was found to be in the second category and according to the amount of nutrient was mesotrophic. The water quality in reservoir was suitable for house-hold consumption when property treated.