Transmission electron microscopy was used to the cysts and their walls of Sarcocystis arieticanis within cardiac muscle of naturally infected sheep. The general ultrastructural features of the cysts, previously described by other authors, were confirmed. The cyst walls of S. arieticanis and the size and shape of the protrusions at different locations on the cyst wall were explained in detail. The cysts were 35-62.5 μm × 52.5-162.5 μm in size and the cyst wall had different-shaped protrusions. Aged cysts that were localized in the heart and differences in the morphology of their protrusions were noticed.