As part of a study on livestock productivity under trypanosomosis risk in the region of Boundiali, northern Cote d'Ivoire, 21 herds of cattle (N'Dama, Baoule and Zebu crosses) and 20 flocks of Djallonke and Djallonke x Sahel sheep were monitored monthly for body weight, packed red cell volume and trypanosomal parasitaemia over various periods between January 1984 and December 1992. A tsetse control campaign using biconical traps impregnated with alpha-cypermethrin started in December 1987. Tsetse control reduced the relative tsetse density by over 95 percent between 1988 and 1992, and this was associated with reductions in the prevalence of Trypanosoma congolense over the same period of over 90 percent both in sheep and cattle. Average reductions in the prevalence of T. vivax were lower, on average 68 percent in adults and 85 percent in young animals. Attempts were made in the design of the study to allow comparisons between controlled and uncontrolled areas; however, there were too many confounding and uncontrollable factors to allow such comparisons to be made. It was necessary, therefore, to compare data collected from all herds and flocks before and after the intervention, with the consequential difficulties in accounting for uncontrollable year-to-year variations in factors affecting trypanosome prevalence in livestock.