Bovine tropical theileriosis (Theileria annulata infection) is of great economic importance to several countries of Asia and Africa where millions of cattle are at risk of exposure to this disease. The authors describe symptoms, sequence of appearance of the parasite and pathology of the infection by three virulent strains of the parasite in fully susceptible calves. Symptoms observed were: enlargement of local lymph gland, fever, haemorrhages on visible mucous membranes and sometime on skin, anaemia and jaundice particularly in subacute and chronic forms of the disease. The parasite "schizont" was first detected in the swollen local lymph gland draining the site of attachment of ticks, then in other lymph glands which concurred with the onset of fever. Main post-mortem findings were: oedema and enlargement of, and haemorrhages in lymph glands and spleen, haemorrhages in subcutis and on most of the serous and mucous membranes on endocardium, pericardium and epicardium, and ulcers in in abomasum which rarely extended to intestine, oesophagus, tongue and gums. The microscopic lesions were characterized by hyperplasia of lymphoid cells at the haemopoietic centres in lymph glands and spleen followed by regression and degeneration of the germinal centres. Infiltration by lymphocytes and macrophages was observed in intermysial (heart) and periportal (liver) areas in interstitial spaces in kidneys. T. annulata parasitises lymphoid cells. It provides stimulus for accelerated production of the cells which results in hyperplasia of the lymphoid tissue. Increased activity of the reticular tissue is a protective phenomenon.