One thousand, nine hundred and ninety-seven male and 1988 female Glossina morsitans submorsitans were dissected at Ogbomosho, on a trade cattle route in south-western Nigeria, from June 1970 to August 1973. Of male flies, 1307 (65-45%) were infected by Trypanosoma vivax trypanosomes, 66 (3-31%) by the subgenus Nannomonas (congolense group) and three (0.15%) by the subgenus Trypanozoon (brucei group). Of flies, 1236 (62-17%) had T. vivax infections, 80 (4-02%) had infections of the subgenus Nannomonas and two (0.10%) had infections of the subgenus Trypanozoon, The great majority of T. vivax infections were mature, while a high proportion (10% in males and 22% in females) of infections of the subgenus Nannomonas were immature. No infection with the subgenus Trypanozoon was found after October 1971. Overall infection rates, for male and female flies respectively, rose from 77% and 80% in June 1970 to peak values of 91% ad 90% in April and June 1971 and thereafter declined to lowest values of 37% and 43% in 1973. The incidence of T. vivax infections was the major component of the rises and falls in overall infection rate. During the period of peak infection rates (April-June 1971), all female flies over about 40 days old were infected. It was assumed that all these had T. vivax infections; some also had subgenus Nannomonas infections and one also had an infection of the subgenus Trypanozoon. Of 72 specimens of G. tachinoides dissected at Ogbomosho from June 1970 to August 1972, 20 (27-8%) were infected by trypanosomes, 19 with T. vivax and one with immature subgenus Nannomonas. In this species, also, infection rate was related to the age of the fly. Of 43 specimens of G. m. submorsitans dissected at Ilorin, north of Ogbomosho on the same cattle route, in February 1975, seven (16-3%) had T. vivax infections. One of two specimens of G. tachinoides dissected here at the same time had an immature infection of the subgenus Nannomonas. In G. m. submorsitans, variations in age structure of the population did not account for the temporal fluctuations in infection rates. It appeared that since 1970 events further north, especially tsetse eradication and the natural decline of tsetse populations (due probably to drought) reduced the trypanosomiasis risk to cattle proceeding southwards. The declining infection rates at Ogbomosho, after 1971, reflected this.