Abstract: Seasonal observations were made on phytoplankton species compsition,their quantitative distribution and their relations with environmental facters in Lake Baiya-ngdian after storing water from May 1990 to July 1992.Four hundred and eight.een species of phytoplanktons were identified,of which 191 species and 8 varietiesbelong to 65 genera of Chlorophyta, 98 species and 11 varieties to 27 genera of Ba-cillariophyta,and 68 species to 27 genera of Cyanophyta,The dominant species were Selenastrum bibraianum,Cryptomonas marrssonii,Dinobryon scrtuldaria,Frag-ilaria crotonensis,Chiorella vulgaris and Scenedesmus bijuga in spring,C. ovaia,Os-cillatoria amoena,F. virescens,Closterium tumidum and Dactylococcopsis acicularis in summer,and O. formosa,Tribonema vulgare and Ulotherix miniliformis in aut-umn,The maximum density of 973.7?04 cell/L occurred in Nanliuzhuang lakeregion and minimum density of 26.0?04cell/L in the Hancun lake region. Theannual mean cell density in the whole lake including eight lake regions was 291?04 cell/L. The maximum density of 411.8?04cell/L was observed in spring,andchlorophyta was most abundant, with an average density of 353.2?04 cell/L. Thediversity indices of phytoplankton composition ranged from 0.6 to 0.9(Simpson'sindex)and 3.1to 4.5(Shannen-Weaver's index).