Abstract: A ubiquitin encoding cDNA from Trypanosoma cruzi, the protozoan cause of Chagas' disease, was isolated by immunoscreening a lambda gt11 expression library with serum from a mouse chronically infected with this parasite. The cDNA encodes a precursor protein consisting of four tandem repeats of ubiquitin differing from that of Saccharomyces cerevisiae in four positions, followed by an unrelated 52-amino acid tail containing a putative metal and nucleic acid binding domain. Southern and Northern blots of DNA and RNA from various strains of T. cruzi and from several African trypanosome and Leishmania isolates revealed dramatic differences in the numbers and sizes of ubiquitin genes and transcripts. One of the T. cruzi isolates examined has as many as 10 ubiquitin genes, while a strain of Leishmania donovani appears to have only 1. Forty or more tandemly arranged ubiquitin coding repeats are present in some genes, while others have only two or three. Evidence for stage-specific expression of a ubiquitin transcript was found in one strain, but no stress-induced changes in the pattern of transcripts were detected in the one isolate examined. Thus the cellular requirements for ubiquitin in trypanosomatids can be supplied by diversely organized genes containing highly variable numbers of ubiquitin coding repeats.