The aim of this study was to examine the clinical and pathological findings in sheep naturally infected with severe encephalitic sarcocystosis and to evaluate the prophylactic effect of amprolium on the disease. From a flock of approximately 350 animals, 10 sheep were referred to the Veterinary Faculty Clinic with neurological symptoms that developed during the previous 2 weeks. These 10 sheep were clinically and pathologically examined, and the remaining animals in the flock without neurological symptoms were treated. All 10 animals were hypothermic, weak, and unable to stand. Complete blood count and serum biochemical analysis revealed mild leukocytosis, hypoglycemia, hyponatremia, hypochloremia, hypocalcemia, and hypoalbuminemia. Gross lesions were not observed during postmortem examination of the central nervous system (CNS). On histopathological examination focal areas of perivascular cuffing in the brainstem, cerebellum, and medulla spinalis that were comprised mainly of lymphocytes, monocytes, and plasma cells were observed. The lesions were associated with Sarcocystis sp. schizonts. Serological analysis for Toxoplasma gondii was negative. The animals in the flock that did not show neurological signs were prophylactically treated with amprolium 10 mg kg(-1) day(-1) for 5 days. No new neurological symptoms were seen in the flock after treatment.