Seasonal changes and the vertical distributions of the water quality in Kawahara Lake were observed during Feb.-Oct. 2000. With progressing of thermal stratification in the lake, DO decreased gradually in lower layer and the nutritive salts (NH₄-N, PO₄-P) released from bottom sediment.<br>On the other hand, in the middle layer of 4-5m depth, phytoplankton biomass grew and aggregated densely in the state of thin layer about 1m thick. The dominant species of the phytoplankton in that layer is Gonyostomum semen. It was concluded that the limiting factor for the biomass growth was NO₃-N, because the bloom occurred after inflowing of NO₃-N with heavy rainfall.