Abstract: Indoor freshwater microcosms were constructed to mimic pond ecosystem near paddy rice area in Zhuji,Zhejiang Province,southeast of China.Single application of chlorpyrifos at 50 μg/L and 5 μg/L to study insecticidal effects at ecological level.Result of the study indicated that the zooplankton community was seriously affected at both concentrations.The order of sensitivity of the zooplanktons towards chlorpyrifos was Cladocera sp.>Copepoda sp.>Rotifer sp. .The most sensitive group was Cladocera sp. .Abundance of the Cladocera sp.did not recover at high concentration till the end of experiments.Chlorpyrifos produced little effect on the Copepoda sp. .The population decreased immediately after application but it increased after 14 days.The Rotifer sp.was the most insensitive one to chlorpyrifos,and due to the removal of the competitive stresses,the abundance of the treated groups rose beyond controls.Result of the PRC analysis showed that the ecological impact of chlorpyrifos was concentration-dependent,and the microcosms did not recover in 35 days after application.