Abstract: Monthly observations were made on the species composition,quantitative distribution of phytoplankton and their relation with the environmental parameters after controlling water pollution in the West Lake from February 1985 to Feb.1986.229 species were identified. Among them,39 species belong to 19 genera of Cyanophyta;87 species,8 varieties and 1 forma belong to 35 genera of Chlorophyta;54 species,7 varieties and 1 forma belong to 24 genera of Bacillariophyta.The dominant species are Aphanizomenon flos-aquae,Raphidio- psis curvata,Spirulina major,Lyngbya contorta,Microcytis aeruginosa,Merismopedia tenuis-sima,Anabaena affinis,Fragilaria crotonensis,F.construens,Melosira granulata Besides, Scenedesrnus spp.and Dinobryon divergens were also abundant.The annual mean cell density in the whole West Lake including six lake regions was 614?05 cells/L.The ma-ximum density of 529.1?05 cells/L occurred in the Waixihu region and minimum density of 1.76?05 cells/L in the Santanyinyue region.Cyanophyta was most abundant,having an average density of 423?05 cells/L.Density of 97.3 and 77?05 cells/L were observed for Chlorophyta and Bacillariophyta respectively.A strong negative correlation (r=-0.92) was found between cell density and transparency.The similarity indices of phytoplankton com- position between the stations ranged from 51.3 to 85.3.