Abstract: Phoxim resistance in a resistant Phoxim-R strain of Helicoverpa armigera was introgressed into a susceptible BK77 strain through repeated backcrossing and selection with phoxim. The introgressed strain (BK77-R) and the BK77 strain were a pair of near-isogenic strains. The BK77-R strain not only developed 155-fold resistance to phoxim compared with the BK77,but it also obtained high level cross resistance to deltamethrin (resistance factor,RF 248-fold),middle level cross resistance to methomyl (RF 31-fold) and endosulfan (RF 11-fold),and low level cross resistance to profenofos (RF 4-fold). S,S,S-tributylphosphorotrithioate (DEF,esterase inhibitor) had significant synergism to phoxim (synergism ratio,SR 7-fold),methomyl (SR 2-fold) and endosulfan (SR 1.9-fold). Piper-onyl butoxide (PBO,oxidase inhibitor) had synergistic effect against deltamethrin (SR 21-fold), methomyl (SR 2.2-fold) and phoxim (SR 1.7-fold). The oxidase and esterase activity of the BK77-R strain was significantly higher than that of the susceptible BK77 strain,and the glutathione S-transferase activity was not significantly altered. The results demonstrated that esterase-mediated metabolism plays an important role in phoxim resistance in the BK77-R strain,and enhanced detoxification mediated by both esterases and oxidases is an important factor conferring broad cross-resistance to different classes of insecticides.