Male pupae of the tobacco hornworm, Manduca sexta (Linnaeus), were exposed to γ–radiation from a 60Co source. This treatment induced mortality and malformations in the emerging adult insects. The incidence of these adverse effects of radiation declined with increasing pupal age at the time of treatment. Within any one age class, the effects of radiation treatment increased with increasing dose. When 16 d old pupae were irradiated, the resulting adult male insects exhibited adverse effects on mating behaviour and fertility when tested against unirradiated females. These effects were dose dependent in the range 0–30 krad. The major cause of reduced fertility was the failure of eggs to hatch. These eggs mostly showed no sign of embryonic development. F1 progency of moths given substerilizing doses of radiation showed significantly elevated larval mortality, pupal and adult malformations, and a slightly prolonged developmental period, but nevertheless a high proportion of the F1 insects survived to adults. The sex ratio of the F1 generation was markedly skewed in favour of males. When F1 insects, male or female, were mated with normal insects of the opposite sex, they were significantly less fertile than the irradiated parental (P1) generation. The F1 male moths transferred spermatophores to normal females only slightly less well than normal males, but the F1 females were much less successful than controls in mating with normal males. Examination of spermathecae of normal females mated with F1 males showed that both eupyrene and apyrene spermatozoa were successfully transferred from the spermatophore in all cases where the P1 insects had received 10 krad, but there was a slight reduction in the cases of higher doses. There was a dose-related decline in the number of eggs laid per mated female, and a dose-related reduction in hatching rate for the matings of both male and female F1 insects, but the effect on males was more severe than on females. The longevity of both F1 males and F1 females was reduced. The reduction in fertility of the F1 insects was in part due to a reduction in the number of eggs laid, but also to an increase in the number of eggs which failed to hatch. In contrast to the P1 generation, the reduction in hatching success of eggs laid by F1 insects was due mostly to the death of partly developed embryos.