On isolation and characterization of Leishmania parasites from Sudanese patients with visceral leishmaniasis (VL), four cases of mixed infections were found. Three of those cases were from the Eastern Sudan focus of VL. In one case the patient was found to be concomitantly infected with Leishmania donovani and Leishmania aethiopica, while the remaining three patients possessed mixed infections of Leishmania donovani and Leishmania major. Mixed infections were identified by PCR amplification of Leishmania kinetoplast DNA (kDNA) from parasites in culture or in original patient aspirate material and, additionally in the former cases by isoenzyme electrophoresis. In those cases where parasite culture was successful, PCR also demonstrated the rapidity with which one Leishmania species was eliminated from culture during continuous passage.