Antigenic variation of African trypanosomes has been reviewed repeatedly in the past 1 0 years. We therefore expect the reader to know that bloodstream trypanosomes are covered with a coat of variant-specific surface glycoprotein (VSG), that there are some lo3 VSG genes in the trypanosome genome, that these genes can move into an active, invariably telomeric, expression site (ES) by duplicative transposition, and that there are multiple ES, which appear to be independently controlled. For details see [ 1-51. Antigenic variation involves two types of VSG repertoires: ( i ) the metacyclic repertoire that develops in trypanosomes in the salivary gland of the tsetse fly; and (ii) the bloodstream repertoire that develops shortly after the trypanosome enters the mammalian host. Although the same VSG genes can be expressed as part of both repertoires 161, it is now clear that the metacyclic repertoire uses another type of ES than the bloodstream repertoire. T h e metacyclic repertoire and its ES have been discussed by Dave Barry and co-workers [6a]. Here we discuss the bloodstream repertoire and focus on one question: how are ES switched on and off'?