African trypanosomes change their antigenicity by successively expressing different members of a group of related but highly diverse proteins, the variant surface glycoproteins (VSGs). We describe a synthetic oligonucleotide that can prime specifically reverse transcription of VSG mRNA out of total trypanosome poly(A)+ RNA. The specificity of this priming was verified by cDNA sequence analysis of the transcription products and by the demonstration of variant-specific hybridization of the individual cDNAs to cellular RNA. The oligonucleotide primer also was used as a probe for the conserved sequence found on these VSG mRNAs in trypanosome genomic DNA libraries. A large number of primer-positive clones were detected in a Trypanosoma gambiense genomic library, but very few positive signals were found in a library of Trypanosoma congolense genomic DNA.