THE unicellular eukaryote Trypanosoma brucei evades the immune defence of its mammalian host by antigenic variation 1. The genes for variant-specific surface glycoproteins (VSGs) are expressed within large multicistronic transcription units 2. Mature messenger RNAs are produced by trans-splicing and polyadenylation 3-5. A remarkable feature of the transcription of VSG genes is its insensitivity to the RNA polymerase II inhibitor alpha-amanitin 6. This has led to the speculation that RNA polymerase I, normally only involved in the transcription of ribosomal RNA genes, also mediates expression of these surface antigen genes. In higher eukaryotes, however, transcripts produced by RNA polymerase I were found to be poor substrates for processing into mature mRNAs 7-9. In contrast, we show here that the RNA polymerase I of T. brucei can mediate the efficient production of functional mRNA for neomycin phosphotransferase. The exceptional ability may be related to the unusual way in which pre-mRNAs are capped in trypanosomes. In most eukaryotes, mRNAs are modified at their 5' end by a capping activity associated with RNA polymerase II10; in trypanosomes, mRNAs acquire their 5'-cap from capped mini-exon donor RNA by trans-splicing 3-5, a process that could be independent of the RNA polymerase producing the pre-mRNA.