The infection rates and transmission characteristics of sterile male Glossina morsitans centralis, G. austeni and G. tachinoides for Trypanosoma vivax, T. congolense and T. brucei were studied. Teneral males were irradiated with 10 krad in a 137caesium source under ambient conditions. Five groups of teneral males of each species were sterilised by irradiation with 10 krad caesium source under ambient conditions and then fed for 3 days, together with five groups of normal males of their respective species, on three goats infected with T. vivax, T. congolense or T. brucei. The mean sterility rates were 98, 93 and 90% for G. m. cenlralis, G. austeni and G. tachinoides respectively. The mean percentage mature infection rates in the irradiated as against the controls of G. m. centralis, G. austeni and G. tachinoides were respectively 27.3 ± 9.5/27.7 ± 11.1, 4.0 ± 1.4/4.0 ± 2.6 and 14.8 ± 7.0/12.2 ± 5.0 for T. vivax; 24.6 ± 5.2/30.3 ± 4.3, 0.7/0.3 and 0.3/0.3 for T. congolense; and, 22.7 ± 2.6/23.4 ± 3.1, 1.1/0.2 and 0.7/1.7 for T. brucei. The transmission rates and frequency to A/J mice were very high and did not differ between the sterile and normal males. There was also no difference in the pattern of the diseases in the goats challenged by sterile or sexually fertile males of the three tsetse species. These results indicate that releases of sterile male tsetse in tsetse control programmes will potentially increase the trypanosomiasis risk in the affected area. Hence appropriate measures should be integrated in any SIR method of tsetse control campaign in order to provide adequate protection to man and his associated livestock.