In order to understand the effects of the ingestion of metazooplankton on the formation of Microcystis blooms in summer in Lake Taihu,a field simulation experiment had been carried out by taking water samples from the Meiliang Bay in Lake Taihu from July 15 to August 14,2009.During the period,visible Microcystis blooms floating on the water surface were found in the controls which did not filter out metazooplankton,but Microcystis blooms were not found in the treatments which had filtered out metazooplankton.Nine species of metazooplankton were found in the controls and three species of metazooplankton in the treatments during the whole experiment.Metazooplankton were found in the treatments in the sixth days of the experiment.The zooplankton biodiversity (H) analysis showed that the value of H in the controls was significantly higher than that in the treatments.During the later stage of experiment (21-30 days),the average density of Microcystis spp.in the controls was significantly higher than those in the treatments.During the experiment,the density of Microcystis wesenbergii and Microcystis flos-aquae in the controls were significantly higher than those in the treatments,and Microcystis wesenbergii contributed 60.79% of the total Microcystis abundance in the controls.The results showed that the ingestion of metazooplankton could not control the Microcystis blooms,on the contrary,metazoop-lankton,especially large metazooplankton,promoted the formation of Microcystis blooms.The experiment also suggested that the community structure of metazooplankton was an important factor in affecting the formation of Microcystis blooms.