Using Tetrahymena thermophila as the indication organism, the single acute toxicity and genotoxicity induced by 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP) were determined. The results showed that the 24, 48, 72 and 96-hour EC50 values of 3 individual CPs to T. thermophila were 3.69, 3.54, 3.02, 2.34 mg x L(-1); 3.23, 2.83, 2.56, 1.97 mg x L(-1) and 0.63, 0.45, 0.34, 0.28 mg x L(-1), respectively, which revealed that the acute toxicity of CPs increased with the number of chlorine on benzene ring. Effects of water hardness on the acute toxicities of 2,4-DCP, 2,4,6-TCP and PCP to T. thermophila were evaluated. It showed that the effect of water hardness on the acute toxicities of the three CPs to T. thermophila had a certain influence, but the impact was relatively insignificant. The results in the SCGE showed that there were significant differences (P < 0.05) between the control and treatment groups at various concentrations of the three CPs. It recommended that the comet assay of T. thermophila was sensitive to the three detected CPs and could be a useful tool for screening and detecting of environmental genotoxins.