The genes encoding procyclin, the major glycoprotein expressed on the surface of procyclic forms of Trypanosoma brucei, comprise a multigene family. It has previously been demonstrated that procyclin genes in cloned trypanosome strains from Kenya and Uganda show restriction fragment polymorphisms. A detailed study of the Kenyan strain 227 has revealed that procyclin genes are arranged in tandem at 3 distinct loci (Pro A, B and C) and that the polymorphism is due to the duplication of 1.3 kb in the Pro A locus, which has generated an additional procycin gene. Northem blot analysis has shown that at least 2 loci are transcribed and that a minimum of 3 procyclin genes are expressed within a cloned line. The transcription of procyclin genes is resistant to lmg ml-I tx-amanitin, whereas that of the 5' flanking gene in the Pro A locus is sensitive. This observation suggests that the two genes form part of separate transcription units with a promoter between them.