The longevity of adult male tropical spotted cutworm,Amathes c‐nigrum(L.) was not significantly affected when irradiated with 20, 30, 40 or 50 krad in air, and 50, 60 or 75 krad in nitrogen. However, the longevity of treated females was significantly increased at 40–50 krad in air and 50–75 krad in nitrogen. Fecundity was significantly reduced when males or females were treated with 20 krad or above in air and 50 krad or above in nitrogen and then crossed with untreated adults. Oviposition by untreated females, mated with treated males at 50 krad in air or 75 krad in nitrogen, was reduced by about 53%, while in treated females crossed with untreated males it was about 96%, at either dose levels or irradiation atmosphere. Per cent egg hatch was significantly reduced at 20 krad or above in air and 50 krad or above in nitrogen when the treated males or females were paired with untreated adults. Complete sterility (no egg hatch) was induced in treated males at 50 krad in air and 75 krad in nitrogen, while treated females were rendered sterile at 40 and 60 krad in air or nitrogen respectively. The number of spermatophores was not significantly different at all dose levels, mating combinations or irradiation atmospheres. Females were found more sensitive than males at all levels of irradiation. For a comparable sterility, the dose in air had to be multiplied by a factor of 1.5 for irradiation in nitrogen.