Abstract: The effect of 5% fipronil flowable formulation and 95% malathion pure chemical on topmouth gudgeon ( Pseudorasbora parva ) was studied. The toxicity of 0.08 mg/l fipronil and 7 mg/l malathion mixture to topmouth gudgeon was greatly improved compared to the single 0.08 mg/l fipronil or 7 mg/l malathion. The potentiation toxicity index was 1000,much greater than 20,the threshold. After 48 hours administration, the AChE activity from the 0.01 mg/l fipronil and 1 mg/l malathion mixture treatments was nearly the same as that from the single 1 mg/l malathion treated .During recovery period, the former was lower than the latter and after 144 hrs, 192 hrs, the difference was significant (p<0.05) respectively. This enhances the unsafety for fish to live in the environment where there is continuous or repeated AChE inhibitors. The significance of recovery study was also discussed.