Members of the Trypanosomatidae, which include the African trypanosomes, the American trypanosomes and the leishmanias, cause disease in vast proportions in man and his livestock and are a major detrimental factor to the social and economic well-being of the third world. Current research using the techniques of molecular biology has revealed two unusual types of mRNA processing in these protozoans; these are the addition of a shared leader sequence to the 5' ends of nuclear mRNAs by a mechanism of trans splicing, and the insertion and deletion of specific uridine residues in mitochondrial transcripts by RNA editing. The presence of these two mRNA processing pathways in the Trypanosomatidae has profound consequences for the organization and expression of their genetic information.