Abstract: The 3rd instar larvae of methoxyfenozide-resistant(R) and homologous susceptible(S) populations of Helicoverpa armigera were treated with sublethal concentration(LC40) of methoxyfenozide for 48 h,and then the specific activities of mixed function oxidases(MFO),general esterases(EST) and glutathione S-transferase(GSTs) in 3rd,4th,5th and 6th instar larvae were measured.The results showed that,for S population,the activities of EST increased in 3rd,5th and 6th instars, especially in the 3rd instars, the activities of GSTs significantly increased only in the 5th instars,while the activity of MFO decreased in the 6th instars.For R populations,the activities of EST obviously decreased in the 3rd and 6th instars,and both activities of GST and MFO decreased significantly in all tested instars.Comparing with S population,the activities of GSTs of R population increased in 3rd-6th instars,but the activities of EST and MFO significantly increased only in the 3rd and 5th instars,respectively.The results suggested that the resistance of H. armigera to methoxyfenozide was closely related with increasing activities of GSTs; the activities of EST and GSTs in S population could be induced by LC40 of methoxyfenozide; but the three enzymes'activities were inhibited in R population at the same concentration.