An immature impala (Aepyceros melampus), captured in the Machakos district of Kenya, had Theileria-like parasites in its erythrocytes. Blood from this animal was inoculated into a Theileria-free impala born in captivity and a Theileria-free, splenectomized steer. Theilerial piroplasms were detected in the erythrocytes of the recipient impala 10 days after inoculation, and a peak parasitaemia of 2% was reached after 48 days, but no infection was detected in the steer. Anaemia developed in the recipient impala in association with the theilerial infection. The morphology of the theilerial piroplasm is described. Possibly dividing parasites were identified. Attempted tick transmission of the impala Theileria to a steer using the ixodid tick, Rhipicephalus appendiculatus, was unsuccessful. With an antigen prepared from the impala blood and the indirect fluorescent antibody test, this parasite was serologically distinct from other theilerial parasites isolated from Bovidae in East Africa.