The Theileriidae of East African wild mammals are reviewed. Three species of wild Bovidae were captured in East Africa. They were Africa Cape buffalo (Syncerus caffer), blue wildebeest (Connochaetes taurinus) and eland (taurotragus oryx), and all were found to be naturally infected with Theileria species. These animals were studied to determine the transmissibility and pathogenicity of their theilerial infections to cattle. Adult Rhipicephalus appendiculatus ticks, which had engorged as nymphs on buffalo, transmitted fatal Theileria lawrencei infections to cattle. African buffalo were shown to be continually infective for ticks over a period of many moths, demonstrating that buffalo can remain a carrier of T. lawrencei. In contrast, attempts to transmit the Theileria of wildebeest and eland to cattle through rhipicephalid ticks failed, despite the establishment of these parasites of the ticks. The significance of these results is discussed in relation to the epidemiology of bovine theirlerioses. During these studies, Anaplasma marginale was transmitted by blood passage from wildebeest to splenectomized calves.