Abstract: The characteristics of action,persistent toxicity and enzyme induction of fipronil on rice stem borer,Chilo suppressalis were taken into investigation.Results showed that fipronil had powerful insecticidal activity against the larvae of Chilo suppressalis,but the sensitivity of the larvae to the toxicity of fipronil was gradually decreased along with their growth.However,fipronil showed lower inhibitory activity against pupae eclosion and no activity against egg hatchability.Fipronil with sublethal doses could persistently induce the larvae dead,and decrease the rate of weight increasing of survival larvae, even the weight of succedent pupae.After treated by fipronil with sublethal doses,the activity of carboxylesterase in 4th instar larvae was induced and kept higher than the control except for the larvae metamorphosis;glutathione-S-transferases activity was significantly improved in 3 hours from the drug application;and the activity of microsomal multifunctional oxidases was enhanced up to 1.37 times higher than the control during the whole experiments.