Abstract:In order to explore the stabilities of resistance to Bacillus thuringiensis (Bt) Cry1Ac toxin in cotton bollworm Helicoverpa armigera (Hübner) and its changes of relative fitness, the resistance regressions of Cry1Ac-resistant strain after transferring to normal artificial diet and its resistance recovers after re-selection were determined using bioassay test, the differences of relative fitness among susceptible, resistant and regressive strains were also compared by measuring their biological characters. The results showed that the resistance of resistant strain (BtR, which had high resistance to Cry1Ac) decreased very rapidly when lost selection pressure, the resistance ratio reduced from 3626.67-fold to 1436.67-fold after 4 generations, the resistance ratio reached to less than 10-fold till 12th generation, after that it kept a relative stable low resistance level (BtR-D, resistance ratio was 6.67-fold). While the H.armigera in this BtR-D strain was re-selected using Cry1Ac, the resistance regained quickly and the resistance ratio in- creased to 1123.33-fold after reselection for 5 generations. Compared with susceptible strain (96S), the fitness of BtR was obviously lower, the relative fitness was only 0.33. However, after continuously reared on normal artificial diet for 14 generations, the fitness obviously increased, the relative fitness was 0.87, the major features were egg hatchability and survival rate of larvae were enhanced.