Hepatozoonosis was studied in hyaenas, lions, jackals, cheetahs and one leopard in the Kruger National Park and compared with the condition seen in dogs in the Republic of South Africa. Hepatozoon schizonts were found in the wild carnivores. The genesis of microschizonts was followed and is illustrated. The schizonts were sometimes very plentiful in the lung, myocardium and skeletal muscle, and were also encountered in the spleen, liver and lymph nodes. Gametocytes were present in leucocytes. The host response was usually very mild. Sporogenous development in ticks was observed in Rhipicephalus simus females removed from an infected hyaena and R. sanguineus adults fed on an infected jackal in the nymphal stage. Attempts to transmit Hepatozoon from a jackal to dogs by means of ticks gave inconclusive results. The developmental stages of Hepatozoon seen in dogs were very similar to those encountered in wild carnivores, except for the presence of more prominent residual material in the mature microschizonts. Schizonts were found in the spleen, liver, lungs and lymph nodes. In most cases the host response to the schizonts was mild and the parasite appeared to be of little consequence to the animal. Nevertheless, severe lesions were sometimes found in uncomplicated cases as well as those complicated by intercurrent diseases such as babesiosis and viral infections. In dogs with severe infections there was considerable necrosis and a marked reticulo-endothelial response with granuloma formation in the spleen, liver and lymph nodes.