Abstract: The relationship between lab acute toxicity and field toxicity of herbicide cyhalofop-butyl on Rana limnocharis tadpole have been studied. The difference between lab toxicity and field toxicity was explained by monitoring the degradation dynamics of cyhalofop\|butyl in field. The acute toxicity test showed that the 24 h-LC50 and 48 h-LC50 of cyhalofop-butyl on Rana limnocharis were 0.718 and 0.677 mg·L-1, respectively, and the toxicity level could be categorized as high toxic. The simulative field trail showed that the death rate of Rana limnocharis tadpole is 86.7% with the dose 1 500 g·hm-2 of cyhalofop-butyl 100EC. The cyhalofop-butyl degraded rapidly in field water .With the dose 1 500 g·hm-2,the water concentration of cyhalofop-butyl was 0.112 mg·L-1, much lower than the concentration of 42.9 mg·L-1 in theory two hours after administration. The results showed that cyhalofop-butyl had low risk to Rana limnocharis though it showed high acute toxicity to Rana limnocharis in lab．

Key words: cyhalofop-butyl, Rana limnocharis, toxicity, risk