Necropsies were conducted on 100 hippopotami during the winter of 1964 in an area of the Kruger National Park where the continuous drought, over a period of three years, had reduced food supplies to a very low level. Systematic observations revealed the presence of 12 helminth species belonging to the classes Trematoda, Cestoda and Nematoda, a blood sucking annalid, a single case of Hepatocystis hippopotami infection as well as a specifically unidentified unicellular organism in the liver and uterus. The latter is believed to have been a coccidium, the bodies observed being unsporulated oocysts having a spinous and striated wall. A summary of the significant pathological changes produced by Schistosoma hippopotami, Fasciola nyanzae and Echinococcus granulosa africanus is presented in Table 1. The pathogenicity of the remaining parasites appears to have been mild or insignificant. The most striking observations were the high incidence and unusual distribution of S. hippopotami, and the pathognomonic cardiovascular lesions in response to adult parasites. These were characterized by the formation of a pseudo-intima in branches of the pulmonary artery and various veins and a pseudoendocardium within the heart. Significant numbers of S. hippopotami were found in the heart and all major blood vessels. Lesions, encountered in the systemic and visceral circulation, were most prevalent in the heart, pulmonary artery branches, posterior vena cava, venae hepaticae and portal veins. Taxonomic affinities of S. hippopotami are presented. 587 The unusual distribution of the adults, the very low degree of ovigenesis and the relatively high number of free and encapsulated schistosome ova in the adrenal cortex, medulla and vein as compared to those of the lung, liver, alimentary tract and pancreas, as observed during winter, cause doubt whether the hippopotamus is a normal host of S. hippopotami. Suggestions made that the hippopotamus could be an aberrant host would require proof that ovigenesis is not subject to a seasonal periodicity, and evidence for the existence of an alternative normal host.