Abstract: The cotton aphid,Aphis gossypii Glover,was continuously selected with imidacloprid in the laboratory,and its resistance was found to increase by 20.03-fold after treatment for 25 times.Field survey revealed that the imidacloprid resistance in cotton aphid varied dramatically with geographical populations.Nanjing aphid population was the most susceptible,and the relative resistance ratio of the aphids from Anyang,Taian and Beijing were 2.21,7.63 and 9.53,respectively.Analysis of detoxification enzymes demonstrated that the glutathione-S-transferase was similar in activity in the resistant and the susceptible strain (activity rate R/S 1.12 folds). However,the activity of esterase in resistant strain was significantly higher than that in the susceptible strain (1.71 folds).Synergism tests showed that DEM had little synergistic effect on imidacloprid in both resistant and susceptible strains (synergism ratio,SR 1.11 and 0.95,respectively).However TPP and PBO showed obvious synergism,and it was more significant in the resistant strain (SR 2.13 and 1.74,respectively) than that in the susceptible strain (SR 1.24 and 1.29,respectively).In addition,treatment with imidacloprid could obviously enhance the esterase activity of aphid populations.Thus,it was concluded that cotton aphid has the potential at least to develop moderate resistance to imidacloprid.The enhanced esterase and cytochrome P450-monooxygenases detoxification might contribute to the imidacloprid resistance in the pest.