SummaryAntigenic variation has been demonstrated throughout a chronic infection ofTrypanosoma brucei(derived from TREU 667) in the mouse. The variant antigenic types (VATs) detected were compared with those appearing on transmission from various time points through the infection. Reversion of VAT was found to occur on transmission throughGlossinaregardless of the VAT(s) present at the time of ingestion. The first patent parasitaemia after transmission is heterogeneous with regard to VAT; three predominant VATs appearing consistently. Indirect evidence suggests that this VAT heterogeneity is most likely not reflected in the metacyclic population but comes about immediately on transmission and can occur in the absence of antibody.