The phytoplankton community in Dae-Chong lake was studied from April to September 1987, for searching the algal blooming observed since 1984. Nineteen species of Cyanophytes, thirty five species of Crypsophytes, fifteen species of Chlorophtes, four species of Pyrrophytes, totally seventy two species were identified in this study. Among them thirty eight species were observed in May and fourteen species in August. Phytoplankton cells were observed 193×10 exp (3)cells/ℓ in June and 2,605×10 exp (3)cells/ℓ in August in average. The dominent species were Asterionella formosa in April, Melosira granulata var. angustissima in May, Straurastrum paradoxum in June, Mycrocystis holsatica in July, Anacystis disperus in August and Anabaena spiroides var. crassa in September. The causal species of algal blooming of Dae-Chong lake was Anabaena spiroides var. crassa. Fifty seven species were observed in Chong-Ju Water Sucking area(Site Ⅰ), forty seven species in Mun-Ui Bridge area(Site Ⅱ), thirty eight species in Dae-Chong Dam area(Site Ⅲ). The biomass of phytoplanktons were 5.582×10 exp (3)cells/ℓ in site Ⅱ, 5,093×10 exp (3)cells/ℓ in site Ⅰ, 4,118×10 exp (3)cells/ℓ in site Ⅲ.