Vaccination against the tsetse-borne trypanosomiases has proved impossible because of the trypanosome's ability to generate a seemingly inexhaustible number of variable antigen types in the blood or tissues of the host. Each variable antigen is a glycoprotein which forms a surface coat on the trypanosome and each glycoprotein is the product of a single gene. The full repertoire of such antigens has not been identified for any trypanosome serodeme (genotype) as yet, but the number of genes coding for variable antigen glycoproteins is estimated to be between 100 and 1,000. We have previously postulated that for Trypanosoma brucei the antigen repertoire of the infective metacyclic stage trypanosomes inoculated by the tsetse fly may be considerably smaller than that expressed in the mammalian host. If this is so then protection against infection by the vector becomes an easier proposition, but the actual scale of the metacyclic repertoire is also unknown. We present here evidence that the metacyclic repertoire of a stock of T. congolense, the most important of the pathogenic cattle trypanosomes, is limited to 12 variable antigen types.