The effect of chemoprophylaxis on developing and mature Trypanosoma vivax in Glossina tachinoides and G. palpalis palpalis was evaluated. Newly emerged G. tachinoides and G.p. palpalis flies were infected with T. vivax by allowing them to feed on parasitaemic animals. Three experiments were conducted and in each the flies were divided into two groups. One group of infected flies was fed on animals treated 1 day previously with isometamidium (1 mg/kg body wt. in a 4% soln.) after which the surviving flies were dissected and examined for trypanosomes. The other group was fed on untreated animals. Out of a total number of 123 flies which fed on treated animals, none were found to be infected, while 51 of 127 flies which fed on untreated animals were infected. It was concluded that prophylactic treatment of animals with isometamidium would eliminate T. vivax infections from the insect vector. The potential significance of this finding in the control of trypanosomiasis in the field is discussed.