Testes from progeny of irradiated maleHelicoverpa zea(Boddie) were dissected and examined cytologically to identify the larval stage or age that would ensure the presence of primary spermatocytes undergoing meiotic division and to determine the lowest dose of radiation necessary to induce visible chromosomal aberrations in 100% of the larvae. Results from this study revealed that cytological examination of chromosomal aberrations could be accomplished best when the larvae were 3- or 4-d-old fifth instars. The percentage of F1and F2males with visible chromosomal aberrations was dependent upon the dose of radiation administered to the P1male. A 6-krad dose was sufficient to induce visible aberrations in all F1larvae. Chromosomal aberrations were observed less frequently in F2larvae than in F1larvae.