In a survey of animal trypanosomiasis in Musoma District, carried out in October and November 1970, 6344 G. swynnertoni were collected from six different localities of the Ikoma-Serengeti area and 623 G. pallidipes from Ikoma. These were dissected and examined for trypanosome infections. The mean infection rates of vivax-type in G. swynnertoni and G. pallidipes were 12.6% and 7.5%, respectively. The congolense group infection rates were 2.0% in G. swynnertoni and 1.8% in G. pallidipes. No salivary gland infection was encountered. The incidence of vivax- and congolense-type infections in general increased with age of both the tsetse species, suggesting that the latter can become infected with these trypanosome types at all ages. The infection rate among female tsetse was higher than among males in the six wing-fray categories. This was due to the slower rate of fraying of the wings with age of the former, so that at each wing-fray category the females were generally older than the males. Vivax-type greatly exceeded congolense-type infection rate in both G. swynnertoni and G. pallidipes. It is suggested that this probably reflects the known greater infectivity of the former group of trypanosomes to Glossina. It is concluded that in Ikoma, where game functions as the reservoir of animal trypanosomiasis and where infected tsetse are abundant, the domestic animals are exposed to a continuous trypanosome challenge.