The metabolic fate of trans-[14C]cypermethrin was studied in Heliothis spp. following topical dosing. A comparison was made between a strain of H. armigera and a strain of H. virescens, both of which are susceptible to pyrethroid insecticides. In sixth instar larvae of H. armigera, more than half of the applied dose was absorbed within 8 hr and most of this appeared as 14C-labelled material in excreta within 48 hr. The major excreted metabolites were the acids trans-3(2,2′-dichlorovinyl)-2,2-dimethyl-cyclopropane carboxylic acid (t-Cl2CA), trans-hydroxy-trans-3-(2,2-dichlorovinyl)-2,2-dimethyl-cyclopropane carboxylic acid (tOH-Cl2CA), and 2-hydroxy-trans-cypermethrin plus 4-hydroxy-trans-cypermethrin (2′4′-OH-t-cypermethrin). Conjugates were also found, and these released 2′4′-OH-trans-cypermethrin and t-Cl2CA after acid hydrolysis. Thus there was evidence of metabolism by monooxygenases and esterases in both species. The comparison of metabolism by the third instar larvae of the two species during the period of 0–8 hr after dosing indicated that H. virescens (i) metabolised a higher proportion of the absorbed dose and (ii) produced relatively more of the metabolite tOH-Cl2CA than did H. armigera. This suggested the presence of a higher monooxygenase activity towards trans-cypermethrin in H. virescens than in the other species. These differences may contribute to the greater toxicity of trans-cypermethrin to H. armigera than to H. virescens.