The variant-specific surface glycoprotein (VSG) genes of Trypanosoma brucei are invariably expressed near the ends of chromosomes (telomeres). We have targeted a VSG gene expression site (ES) promoter driving a selectable marker gene (neomycin phosphotransferase) into a chromosome-internal transcription unit, the tubulin gene array of procyclic trypanosomes. To avoid read through transcription of the marker gene from the tubulin promoter, we targeted the ES promoter in inverse orientation relative to tubulin gene transcription. The only correctly targeted transformant obtained contained the marker gene close to the border of the tubulin gene array, and expression of this gene was relatively low. Possible reasons for the low targeting efficiency and expression level are discussed.