Two Theileria parva stocks were isolated from control cattle during East Cost fever (ECF) field immunization trials at SAO Hill and West Kilimanjaro in the southern and northern parts of Tanzania respectively. Both parasite stocks caused severe clinical ECF which required antitheilerial treatment for 3 of the 5 experimentally infected cattle. Cattle recovering from infection with the two T. parva stocks did not develop a fever and only 1 of 4 animals developed scanty schizont parasitosis for one day during a challenge with T. parva (Kasoba) from northern Malawi. In contrast, both control cattle developed fever and schizonts, and one required antitheilerial treatment to survive.