Two strains of Heliothis virescens, one resistant (PEG87) the other susceptible (BRC-F to insecticide were topically dosed at 3rd instar with trans (1-(14)C cyclopropyl) cypermethrin in acetone. Penetration studies revealed that trans cypermethrin was absorbed at a faster rate in the susceptible (BRC) strain of Heliothis virescens as compared to a resistant one (PEG87). The rapid elimination of radiolabelled metabolites of cypermethrin from PEG87 suggests a higher metabolic turnover of the insecticide in the resistant as compared to the susceptible strain. The major metabolic product in the faeces of both strains was polar conjugated material, with PEG87 larvae excreting more conjugate than larvae of the BRC strain. Analysis of the primary metabolites of trans cypermethrin present in the faeces, larval bodies and haemolymph of each strain indicates that the major route of detoxification is via a monooxygenase which leads to the formation of hydroxylated cypermethrin. The appearance of the products of hydrolysis suggests additional esterase activity although this may be a secondary effect.