One hundred catfish, mud barbel Clarias mossambicus were examined for the presence / absence of Trypanosomes in their blood. The organisms in blood films stained with Gimesa were measured (30 - 40 um in length; 3-5 um in width) and identified as Trypanosoma markewitschi. Fresh blood was obtained from the fish caudal vein. Fourty un-infected fish were inoculated with 0.5 ml whole blood per fish intraperitoneally and kept at 20°C for 5 weeks. By the end of 5 weeks post-inoculation the heart showed myoradial degeneration and interstitial lymphocytic myocarditis. Tissue forms of T. markewitshi were observed either single or double or 4 - 6 large basophilic structures within vacuolated myocardial cells. The spleen had hemosidrosis and formation of large hypocellular follicles occurred. Vascular changes in the liver were experssed by damage of the endothelium lining the blood vessels and perivascular cuffing. The posterior kidneys showed marked cellularity of haematopoietic tissue and vacuolation of the tubular epithelium, which were filled with yellow pigment and suggested bilirubin. These findings were consistent and did not observed in non-inoculated fish and confirmed that T. markewitschi is pathogenic in catfish, despite the low degree of parasitemia.