Abstract: The acute,subchronic and chronic toxicity of pentachlorophenol (PCP) to Daphnia magna were investigated under renewal condition with a hardness of 80-100 mg/L (as CaCO3).At 25-26℃,the first instar (less than 24 h old) D.magna was used in the acute and chronic toxicity tests.The duration of the chronic test was 20 d.At 19-20℃,the subchronic exposure was lasted 19 d for less than 24 h old D.magna while it was lasted 16 d for 4 d old D.magna.The 24 and 48 h EC50 for D.magna were 489 and 245 μg/L,respectively.Depended on the population of the first brood,the LOEC and NOEC were 160 and 80 μg/L in the chronic test,respectively,which were similar to those in 19 d subchronic test (200 and 100 μg/L).The results indicated that the population of the first brood was a sensitive endpoint.