The development of the parasite and lesions was studied in 32 sheep killed 10 days to 47 months after inoculation with Sarcocystis gigantea sporocysts from cats. At 21-42 days post-inoculation (d.p.i.), there was a mild encephalitis, but organisms were not seen in the brain. Immature sarcocysts were detected from 40-84 d.p.i. The cyst wall was not measurable by light microscopy at 40 d.p.i., but was 1.5-2 microns thick at 84 d.p.i. At 119 d.p.i. both immature cysts containing only metrocytes, and mature cysts containing both metrocytes and merozoites, were present. These mature cysts did not have a secondary cyst wall. A mature cyst, 350 microns in length, was found in a sheep killed at 8 1/2 months p.i. At 10 m.p.i. cysts were up to 0.5 mm long and a secondary cyst wall was present. At 47 m.p.i. cysts were 2-5 X 4.5-7.5 mm, and were found only in the muscles of tongue, oesophagus, pharynx and flank.