Résumé : African trypanosomes show an extensive capacity for antigenic variation. The serotype of each antigenic variant is determined by a variant-specific surface antigen (VSA) which forms a continuous coat over the entire surface of the parasite. All known VSAs are glycoproteins whose antigenic specificity is associated with the protein moiety. The expression of some of the VSA genes is linked to the duplication of a 'basic' copy (BC) of the gene and to the transposition of the additional or 'expression-linked' copy (ELC) to a new site, where it is transcribed. Other VSA genes seem to be expressed without being duplicated; they are, however, subject to rearrangements apparently unrelated to their expression. In chronic infections, different variable antigen types (VATs) tend to appear in a loosely ordered sequence: indeed, after either syringe passage or cyclical transmission through the tsetse fly, the same VATs always appear in the early stage of the disease and are therefore known as early or predominant ones. To check whether genes coding for predominant VATs can be characterized by some particular feature of their DNA sequence or gene rearrangement, we have studied here the gene coding for AnTat 1.3, which is the most predominant VAT of our Trypanosoma brucei brucei stock. Three points emerged: (1) when expressed, the AnTat 1.3 gene is duplicated and the ELC transposed in an expression site identical to the one described previously for other VSA genes; (2) its BC is located near the end of a DNA molecule, in a region which is very similar to the expression site and undergoes frequent insertions/deletions unlinked to VSA gene expression; (3) this gene does not belong to a multigene family.