Abstract: The third instar larvae of two Plutella xylostella (L.) strains,one was susceptible to spinosad (SS strain) and the other was obtained by selecting the SS strain with LC25 spinosad for more than five generations (Sub-SS strain),were treated with LC25 or LC50 concentrations of spinosad using cabbage leaf dip method and then determined the activities of the detoxifying enzymes.After treated for 6 h,12 h, 24 h,48 h and 72 h,activities of the carboxylesterase (CarE),the glutathione S-transferase (GST) and the mixed-function oxidases (MFOs) were investigated,respectively.The results showed that the activities of the three enzymes changed with time and different treatments.The specific activity of CarE in SS strain fluctuated significantly both in the control and LC25 or LC50 treated groups.At the beginning of spinosad treatment,the specific activity of GarE increased but was inhibited gradually with the increase of treatment time.In general,the CarE activities of different treatment in Sub-SS strain were higher than those in SS strain.The GST activities were induced significantly by sublethal concentrations of spinosad and had an obvious time effect.However,the activities of O-demethylation of MFOs were distinctly inhibited by sublethal concentrations of spinosad.The activities of MFOs in the Sub-SS were lower than those in the SS strain.