Dynamics of pyrethroid resistance in a field population of cotton bollworm (Helicoverpa armigera) was demonstrated by continuous monitoring with twin discriminating dosages, and the influencing factors were also experimentally analysed. Resistance in a field population in China increased rapidly in the 3rd and 4th generations when population density became higher and insecticides were applied repeatedly, then decreased suddenly during over-wlntering and slowly in the 1st and 2nd generations when insecticide spraying was suspended. Resistance increase could be countered by dilution as a result of immigration of susceptible moths from corn fields, which were found to be a natural refuge for this pest in China, The reduction of resistance during over-wintering and the 1st and 2nd generations was affected by the lower fitness of resistant cotton bollworms to low temperature and disadvantages in reproduction. The possibilities of managing the resistance in field populations on the basis of these observations are discussed.