A survey was undertaken to elucidate the epidemiology and pathogenicity of theileriosis in Malaysia. Blood samples of 322 cattle were collected and examined at 20 locations which covered seven states in Peninsular Malaysia. All the blood smears from cattle had Theileria parasites and piroplasms were associated with an intraerythrocytic bar and veil structure. The parasitaemia varied from less than 0.1% to 2.0%. Twenty-three cattle ware considered anaemic based on their haematological parameters, particularly the values of packed cell volume which showed less than 20%. One calf, naturally infected by Theileria parasites, was splenectomized. The peak parasitaemia, or 17.9%, was observed at the 14th day after the splenectomy and there was a marked reduction in packed cell volume. On the 22nd day, the calf developed a transient infection of Eperythrozoon wenyoni. Tick collections were carried out from some grazing cattle on several occasions. Most of the ticks collected were Boophilus microplus, but two nymphs, four adult males and two adult females were of the Haemaphysalis bispinosa that might be a vector of Theileria in Malaysia.