Laboratory and field cage trials were made with male Glossina morsitans orientalis Vanderplank treated with tepa or gamma irradiation to assess the ability of the sterile males to compete with untreated males for normal females when the ratios of treated males to untreated males to untreated females ranged from 4:1:5 to 5:2:5. Irradiation of the pupal and adult stages with 8000 and 15000 rad or 8000 and 12000 rad, respectively, reduced reproduction by 87–100% (mean, 95%). Contact for 60 min on a glass surface coated with 10 mg tepa/ft2 or exposure to 0·25 ml of 5% tepa in a wind tunnel usually produced complete sterility (mean, 99%) in 0- or 2-day-old male flies. Trials in the laboratory and in a small field cage (288 ft2) with chemosterilised flies generally reduced reproduction to near the expected values. Similar results were obtained with male flies emerging from irradiated puparia, but males treated as adults produced somewhat smaller reductions than expected. Unreplicated competitive trials with chemosterilised and irradiated males in a large field cage (8100 ft2) produced considerably smaller reductions in reproduction than expected, suggesting that treated males released in nature might not compete for normal females as readily as untreated males.