Abstract 2 antigenic variants of a strain of Trypanosoma congolense were passaged cyclically through different groups of Glossina morsitans. 4 cyclically transmitted first parasitaemic populations in mice were preserved from an infected fly in each group. All stabilates were compared serologically by a neutralization test. 3 distinct types, designated A, B and C, were identified. The population of types B and C were derived from different flies, populations of type A were derived from both flies.