The community structure of rotifer and ecological assessment of water quality,including E/O,Q_B/T index and the biodiversity index,were studied in Lake Gehu,Jiangsu Province.After two-year monitoring in situ,69 species of rotifer were identified,39 species were pollution indicator species.The dominant species were Brachionus calyciflorus,Asplanchna priodonta,Polyarthra trigla,Filinia longiseta and B.diversicornis.The average annual density was 1 584 ind./L and the average annual biomass was 5.982 1 mg/L.The total density reached the maximum in autumn and biomass was highest in summer.The species biodiversity was positively related to the density and biomass.Rotifer species were similar in three zones of Lake Gehu.There were no significant differences between the north area and the middle area,but there were significant differences between the north and the south area.Biological assessment of water pollution and trophic condition were evaluated by species indicators,biotic index of E/O,Q_B/T and species biodiversity index.The trophic type of Lake Gehu could be regarded as eutrophic.