Amblyomma cohaerens nymphs, which had been collected as engorged larvae from African buffalo (Syncerus caffer) in the Mara region of Kenya, transmitted a theilerial parasite to a steer. Macroschizonts were detected in the regional lymph node of this steer 12 days after tick application and were subsequently detected for 3 days at low levels of parasitosis. The macroschizonts were large and had numerous nuclei, being morphologically unlike any other theilerial schizonts described previously from cattle in Eastern and Southern Africa, but they resembled the macroschizont of T. barnetti Brocklesby 1965 from a buffalo also obtained in the Mara region. Piroplasms morphologically indistinguishable from T. mutans were first detected in the blood of the steer on the sixty-first day after tick application. The steer showed a significant rise in antibody titre to only T. mutans (Aitong) piroplasm antigen, using the indirect fluorescent antibody test. There was no evidence that this parasite was pathogenic. The intraerythrocytic piroplasms proved to be blood-transmissible to a splenectomised calf. Although the parasite showed affinities to T. barnetti and T. mutans (Aitong) definite conclusions regarding its identity must await further transmission experiments.