Abstract: According to the investigation and analysis of phytoplankton in Danjiangkou Reservoir from July 2007 to June 2008,eight phyla including 60 genera and 110 phytoplankton species and varieties were found.Phytoplankton composition was dominated by Cyanophyta(46 species),Chlorophyta(35 species)and Bacillariophyta,with the two former accounting for 42% and 32% of total taxa,respectively.Algae density averaged 4.17×10⁶ cell/L,and ranged from 5.16×103 to 6.10×10⁷ cell/L.Seasonally,phytoplankton showed significant differences in dominant taxa,with Oscillatoria sp.and Synedra acus dominating in spring,Cryptomonas erosa during summer and autumn,and Peridiniopsis niei during winter.Spatially,Synedra acus was the dominant taxa in the Danjiang,Hanjiang and Qushuikou regions,while Peridiniopsis niei dominated in Wuqing.Phosphorus concentration was the main factor controlling algae density in the Danjiangkou Reservoir.Combined with published Danjiangkou Reservoir data,the phytoplanktonic community trend in the reservoir was also analyzed during several decades after damming.Since 1958,the density of phytoplankton in the reservoir has increased by 16 times,and eutrophication of the reservoir has become increasingly serious.Phytoplankton composition changed from Bacillariophyta to Bacillariophyta-Cyanophyta-Chlorophyta type,and then was gradually replaced by Bacillariophyta-Pyrrophyta-Cryptophyta-Cyanophyta.