Significant suppression in the incidence of cyclical development of Trypanosoma congolense, T. vivax and T. brucei occurred in Glossina morsitans centralis maintained on goats immunized with in vitro-propagated uncoated forms of T. congolense, T. vivax and T. brucei, respectively. This was observed when tsetse given a T. congolense-infected feed were subsequently maintained on uninfected immunized goats and also when uninfected tsetse were fed on immunized goats infected with T. congolense, T. vivax and T. brucei. Suppression of infection rates in tsetse was trypanosome species specific, but was independent of the trypanosome stock used for immunization of goats. These findings were reflected in antibody responses to uncoated trypanosomes, as measured by immunofluorescence and the solid-phase immunoradiometric binding assay. Thus, antibody from goats immunized with uncoated trypanosomes of one species exhibited minimal reactivity with uncoated forms of other species of trypanosomes, but showed high levels of activity with uncoated forms of the same or unrelated stocks of the same species. However, in view of the range of hosts upon which tsetse feed, it is open to question whether the use of a vaccine which suppresses trypanosome infection rates in tsetse would have any significant effect in the field.