The fate of the radiolabeled formamidine amitraz in bollworm, Heliothis zea (Boddie), and tobacco budworm, H. virescens (F.), was examined after topical application to early instars (average weight 25 mg) and late instars (average weight 250 mg) and injection into late instars. Amitraz was degraded by both insect species after topical and injection treatment. Metabolism was qualitatively similar in both species; amitraz was converted to BTS-27271 [N'-(2,4-dimethyl phenyl)-N-methyl formamidine], 2,4-dimethyl formanilide, 2,4-dimethyl aniline, and more polar unidentified metabolites. However, quantitative differences were evident. For example, levels of the amitraz metabolite BTS-27271 were higher in internal extracts of bollworms than in tobacco budworms, suggesting that once formed, this metabolite was degraded more slowly in the bollworms. This observation was corroborated in metabolism studies of radioactive BTS-27271. No 2,4-dimethyl aniline was detected in metabolism studies of BTS-27271 in either species, indicating that the small amounts of this metabolite detected with amitraz may have formed directly from amitraz itself and not from BTS-27271 and 2,4-dimethyl formanilide intermediates. There was no evidence for any major differences in the rates of amitraz penetration and excretion between the two species, although penetration and excretion of amitraz equivalents apparently proceeded at a somewhat faster rate in bollworms than in tobacco bud worms. Similarities in the fate of amitraz in these two closely related insect species were more striking than were any of the differences. BTS-27271 was degraded more slowly than amitraz in both insect species.