Abstract: Phytoplankton in two large scale enclosures and one fence(each was 750 m²) in East Lake of Wuhan was studied from September, 1993 to February, 1995.There were 89 genera which belonged to 7 phyla in the experimental area which included two enclosures, one fence and one control.The genus numbers of Chlorophyta, Cyanophyta and Bacillariophyta were the most during the whole year in the experimental area.The genus numbers of Chlorophyta in the enclosures was more than those in the fence's and the control's, there were not obvious difference among the genus numbers of the other phyla in the enclosures and the fence and the control.The genus numbers of phytoplankton decreased in order of season——autumn, summer, spring and winter in the experimental area.The genus numbers of Chlorophyta were the most, Cyanophyta and Bacillariophyta next during the whole year in the experimental area, the fence and the control.While, the density of phytoplankton decreased in order of season——spring, summer, winter and autumn.Generally, the number percentage of Cryptophyta, Chlorophyta, Bacill ariophyta and Cyanophyta was over 95%.The biomass, Chl a and primary productivity of phytoplankton were highly positive correlate, their values decreased in order of season——spring, summer, autumn and winter, The biomass of phytoplankton in every enclosure was large than that in control's, the values of density, Chl a and primary productivity were not obvious difference among the enclosures, the fence and the control.