Pyrethroid resistance in Helicoverpa armigera (Hu¨ bner) Þeld populations was investigated in Benin over several years by using third- and fourth-instar larval topical bioassays. H. armigera was resistant to pyrethroids tested as cypermethrin, deltamethrin, bifenthrin, and fenvalerate. Pretreatment with piperonyl butoxide signiÞcantly decreased the LD50 value of cypermethrin and deltamethrin, and the resistance suppression by this synergist effect was observed. No signiÞcant decrease in the LD50 value was obtained when S,S,S-tributyl phosphoro-trithioate was applied before deltamethrin. In the Þeld, cypermethrinÕs LD50 value varied, and the highest LD50 values were observed during the rainy season, the cotton, Gossypium hirsutum L., crop period, when pyrethroids are extensively used. In the dry season when there was no cotton cultivation, the lowest LD50 values were obtained. However, reversion was never total in the Þeld; resistance did not revert to the level we observed in the susceptible strain. In the laboratory, when Þeld populations were reared in insecticide-free conditions, resistance decreased and total reversion was observed. Results are discussed with regard to insecticide resistance Þtness cost and resistance management strategies.