In this, the first of a series of papers on the epidemiology of bovine trypanosomiasis in the Ghibe valley, southwest Ethiopia, the tsetse populations and their relationships to the prevalence of trypanosome infections in cattle are described. The tsetse challenge to cattle at two sites sites in the area was estimated as the product of tsetse relative density and the trypanosome infection rate in flies. The proportion of feeds taken by tsetse from cattle was also considered. Three tsetse species were detected in the area, Glossina pallidipes, G. fuscipes and G. morsitans submorsitans. A significant correlation (r = 0.60, P < 0.001) was observed between the mean monthly estimates of tsetse challenge due to G. pallidipes and the prevalence of trypanosome infections in cattle the following month at one site, whilst at the other, no significant relationship was observed (P = 0.08). The tsetse density at both sites showed seasonal changes which were related to the monthly rainfall. Finally, variations in tsetse density appeared to be the main factor responsible for variation in tsetse challenge and thus trypanosome prevalence in cattle.