a parasitic protozoan of the genus Sarcocystis of the order Coccidia of the class Sporozoa. There are several dozen species. Sarcosporidia parasitize the muscular tissue and, less commonly, the connective tissue of mammals, birds, and—sometimes—reptiles. In host tissues they form cysts that are 0.5 mm-5 cm long and separated by septa into several parts, each containing numerous uninucleate crescent-shaped cells 8–15 micrometers in length. Formerly called spores, these cells are merozoites and represent the final stage of the asexual reproduction of sarcosporidia.

In 1972 the German scientist M. Rommel and his co-workers demonstrated for the first time that sarcosporidia pass through a typical coccidian life cycle after penetrating the intestines of cats. Sarcosporidia have oocytes similar to those of the genus Isospora, and the ultrastructure of their merozoites is similar to that of other coccidia. Massive infection of pigs by Sarcocystis miesheriana and sheep by Sarcocystis tenella results in a decline in meat quality. Man is sometimes infected by Sarcosystis lindemanni, whose pathogenic significance is unclear.