Colony specimens of the biting midge Culicoides variipennis (Coquillett) were treated with gamma radiation as larvae, pupae, and newly emerged adults at 5000, 10,000, and 15,000 R, and as pupae at 20,000 and 30,000 R. The criterion for the sterility of males was the hatch of eggs deposited by untreated females with which they mated. Males treated at doses above 5000 R were, with few exceptions, 95%–100% sterile in first matings, but males treated at all levels tended to recover fertility in later matings. Recovery decreased as the radiation dose increased but even 30,000 R failed to sterilize completely. Treated females did not recover and rarely laid more than 1 egg batch, though those batches that were laid often had rather low percentages of sterility. With treated females, the number of eggs per egg batch decreased as the radiation dose increased, until at 30,000 R no eggs were laid. Miscellaneous biological information for colony C. variipennis, principally relating to reproductive performance, was obtained in testing the treated specimens and the accompanying controls.