Inter-annual changes of phytoplankton dominant species and biomass in a large shallow lake,Taihu China,were analyzed using the monthly monitoring data covering the period of 1991～2002.The concentrations of total nitrogen(TN),total phosphorus(TP) and phytoplankton chlorophyll-a(Chla) were used to explain the development of eutrophication in the lake.The results showed marked gradients of TN,TP,Chla and biomass in the lake from high values near the northern river input in the inner Meiliang Bay towards lower concentrations in the lake centre.Phytoplankton dominant species shifted from diatoms to cyanobacteria at the end of 1980's when the eutrophication started in the lake.Thereafter,almost no change in dominant species but phytoplankton biomass varied among years.Annual averages of TN,TP,Chla and biomass increased until 1996 declining thereafter possibly because of the controlling of wastewater discharge from the catchment by the local government.Microcystis-domination in phytoplankton assemblages presented the indication of eutrophication in Lake Taihu.Our results suggest that phytoplankton is a suitable ecological indicator for eutrophication development in large shallow lakes.