Actively growing nondiapausing 3rd-, 4th-, or 5th-instar laboratory-reared larvae of the European corn borer, Ostrinia nubilalis (Hübner), were irradiated with gamma rays from a Co60 source. Diapausing larvae collected in the field were irradiated after either 3 or 5 1/2 months of refrigeration. The nondiapausing larvae had too much somatic damage from the irradiation to make this a practical method of borer control. However, the diapausing larvae showed little evidence of somatic damage. Rates of pupation, moth emergence, and mating were nearly normal at levels of irradiation as high as 5000 rad. Numbers of eggs laid and egg hatch were most severely affected at 4000 and 5000 rad. There was an interaction between time to pupation and dose level when diapausing larvae were irradiated and then divided into 3 groups (A, B, and C) based on length of time to pupation. Egg hatch was significantly affected. All levels of rediation affected pupation period A (early) most strongly. Only 4000 and 5000 rad affected hatchability of eggs from pupation period B (middle). Only 5000 rad affected the egg hatch of pupation period C (late). Irradiation of diapausing larvae appeared to affect the motility or viability, or both, of sperm instead of inducing lethal gene mutations.