Abstract: Effects of spinosad on the activities of polyphenol oxidase(PPO) and carboxyl esterase(CarE) in beet armyworm larvae were investigated. Spinosadshowed high toxicity to the third instar larvae of beet armyworm and its LC50 was 0.80 mg/L. 1.0×10-3~0.5 mg/L spinosad inhibited PPO activities more than 50％ in vitro and the high concentration showed stronger inhibitory activity. In vivo 0.1~0.8 mg/L spinosad induced PPO activity increasing in 4 h and then inhibited its activity after 12 h. 1.0×10-3~1.0 mg/L spinosad had no effect on CarE activity in vitro but which was significantly increased when the 3rd larvae was fed with leaves treated with 0.05 mg/L.