An in vitro assay was used to measure the metabolism of cypermethrin in homogenates of pyrethroid-susceptible and -resistant tobacco budworms. Total metabolism was greater in field-collected and laboratory-resistant larvae compared to that in a laboratory-susceptible strain. In addition, metabolsim of cypermethrin was greater in a late- than early-season collection. These results confirm that enhanced metabolism is associated with pyrethroid resistance in this insect. Three metabolites were identified from homogenates of resistant larvae: phenoxybenzoic acid, phenoxybenzyl alcohol and 4'-HO-cypermethrin. Phenoxybenzoic acid was the predominant metabolite produced in crude homogenates from all strains suggesting that hydrolysis of cypermethrin by esterases plays a key role in resistance in these strains.