To study the community structure of phytoplankton and assess the water quality in Lake Baiyangdian,an investigation has been conducted at eight typical sampling sites.This investigation showed that among the total 133 species identified,Chloro-phyta,Cyanophyta,and Bacillariophyta were dominant in the phytoplankton community.In spring, Chlorophyta and Bacillariophy-ta were the dominant phylum, and Chlorella sp., Chroomonas acuta.Uterm.and Microcystis incerta Lemm.were the dominant spe-cies.The density of the phytoplankton ranged from 496×10⁴ to 6256×10⁴ cells/L with an average of 2384×10⁴ cells/L.In sum-mer,Chlorophyta and Cyanophyta were the dominant phylum,and Chlorella sp.,Leptolyngbya valderiana Anagn.and Nephrocyti-um agardhianum Nageli.were the dominant species.The density of the phytoplankton ranged from 318×10⁴ to 4630×10⁴ cells/L with an average of 1785×10⁴ cells/L.According to the comparison between this investigation and the previous two ecological inves-tigations since 2005, the abundance of the phytoplankton has increased significantly.The index of Carlson nutritional status and the dominant genus assessment revealed that the majority water was eutrophic in the Lake Baiyangdian.