This is the first study of Sarcocystis-induced abortion in sheep. Eleven pregnant ewes were experimentally inoculated with 50,000, 100,000, or 500,000 Sarcocystis ovicanis sporocytis from dogs. Eight ewes either aborted, died, or became moribund before term; they produced 15 fetuses, 11 of normal appearance and 4 necrotic. No evidence of intrauterine transmission was obtained. All infected ewes became anemic, inappetent, and lost weight. Ewes inoculated with the greatest numbers of sporocysts exhibited the most striking signs of acute illness. At necropsy of acutely ill ewes the heart was the most severely affected organ, appearing nearly black as a result of hemorrhagic pancarditis. Less hemorrhage was seen in the kidney, liver, spleen, and skeletal muscles. Microscopically, schizonts were found in capillary endothelial cells of most organs 24 to 33 days after inoculation. Ewes surviving the acute illness appeared generally unthrifty and exhibited the additional signs of wool breaking, and nervous disturbances. At postmortem, the heart and kidneys of these ewes were moderately hemorrhagic, and the adrenal glands and mesenteric lymph nodes were enlarged. Microscopically, sarcocysts were found in the heart, diaphragm esophagus, tongue, skeletal and eye muscles, cerebellum, and cerebrum.