Abstract: Using freshwater photobacterium Q67(Vibrio qinghaiensis sp.-Q67) asindication organism, the single acute toxicity induced by 2-chlorophenol, 2,4-dichlorophenol, 2,3,4-trichlorophenol, and the joint toxicity of binary and trinary mixtures in equiconcentration and equitoxicity ratios were determined, respectively. Additive Index and Similarity Parameter were used to evaluate the joint effects. The results showed that EC₅₀ values of 3 individual chlorophenols to Q67were 99.57, 25.19 and 3.42 mg/L respectively, which revealed that the acute toxicity of chlorophenols increased with the number of chlorine on the benzene ring. The mixture effects were significantly higher than the single toxicity of 3 individuals. The 95% confidence intervals were used for 2 assessment indexes to reduce the inaccuracies due to experimental errors. Their evaluation results were almost identical. The joint action of chlorophenol mixtures were identified as being simple additive, except that 2-chlorophenol and 2,3,4-trichlorophenol groupshowed weak synergistic effect. The same assessment results were concluded in both equiconcentration and equitoxicity mixtures.