The sensitivity ofTrichoplusia ni(Hübner) eggs to gamma irradiation depended upon age and dose: no hatch occurred when 0- to 24-h-old eggs were exposed to 4 krad, 24- to 48-h-old eggs were exposed to 30 krad, or 48- to 72-h-old eggs were exposed to 70 krad. Also, when 24- to 48-h-old eggs were treated with 15 and 20 krad, none of the larvae pupated, but when they were treated with 10 krad, 6% pupated; even when these eggs were exposed to only 4–5 krad, the percentage that pupated, that emerged as normal adults, or that mated was lower than in the control. There were little differences in results between eggs irradiated when 24–48 hand 48–72 h old. Eggs 48–72 h old irradiated with 1.5–3.5 krad produced females that were more sterile than similarly exposed males, but the sterility was not sufficient to permit the use of the females in a sterile-insect release program. All Flprogeny of parents from treated eggs were fertile