Clinical, clinico-pathological and serological studies were performed in sheep experimentally infected with Babesia ovis. Acute babesiosis occurred in all the lambs infested with adult Rhipicephalus bursa ticks and in one lamb infested with the larvae. The rate of parasitaemia and the degree of anaemia were not correlated. Decrease in the packed-cell volume ranged from 30 to 40%. Parasitized erythrocytes were not observed to block capillaries in the brain, which explained the absence of nervous symptoms in acute babesiosis. The kidneys were the most severely affected organs, exhibiting acute glomerulonephritis. The lesions observed were suggestive of vascular alteration and vascular stasis, leading to anoxia of the tissues. A disseminated intravascular coagulation (DIC) syndrome was recorded in sheep infected with babesiosis. A marked increase in the enzymes of the transaminase groups, mainly aspartate aminotransferase (AST), was observed. Enzymatic changes (increases in AST, alanine aminotransferase (ALT) and lactic dehydrogenase (LDH) and decreases in sorbitol dehydrogenase (SDH), alkaline phosphatase (ALP) and malic enzyme (MEZ)), decreases in total proteins and albumin, and increases in urea and creatinine might reflect the degree of severity of the damage to the liver and kidney tissues. Most of the lambs (85%) that were infested with larvae, and all lambs infested with adult R. bursa ticks, reacted serologically to B. ovis antigen. The serological reactions following infestation with the larvae occurred much later than those following infestation with the adult stage. The lambs which were infested with larvae showed mild clinical reactions when challenged by infected R. bursa adults, as compared with the reactions to the challenge in naive control animals. The serological findings, in addition to the fact that one splenectomized lamb reacted to larval infestation with acute ovine babesiosis, show that the preimaginal stages of R. bursa can transmit B. ovis, usually causing a sub-clinical disease. It is suggested that infections derived from preimaginal ticks in the winter can preimmunize sheep for the subsequent more severe infections derived from adult ticks in the summer. Furthermore, in the absence of a reliable vaccine against B. ovis, grazing flocks in the enzootic regions should be exposed to the preimaginal stages during their activity period (October-February) before exposure to the adult ticks in spring and summer (April-July).