Cysts of Sarcocystis muris develop within muscle cells and each is bounded by a parasitophorous vacuole membrane. Closely spaced spherical blebs formed from this membrane extend into the muscle cell cytoplasm. A dense substance fills the cavity of the bleb and occupies the vacuolar space immediately adjacent to the membrane. The remainder of the vacuole is filled with a moderately dense matrix within which the parasites develop. At 40 days after infection only metrocytes are present, characterized by their ovoid shape, lightly stained cytoplasm, amylopectin-like granules, and lack of micronemes. Metrocytes divide by a process resembling endodyogeny and eventually produce bradyzoites. By 78 days after infection, at which time the cyst is infective for cats, the few remaining metrocytes are located at the cyst periphery but most organisms are elongated and contain organalles characteristic for bradyzoites including micronemes, dense granules, and amylopectin. Structures indicative of division were not seen in bradyzoites. Rhoptries are few in number. Numerous vesicles of smooth endoplasmic reticulum accumulate in the cytoplasm of muscle cells adjacent to the periphery of the enlarging cyst but significant destruction of muscle fibers containing cysts with viable organisms was not seen in specimens fixed between 40 and 325 days after infection. Unusual lamellar structures were seen in some parasitized muscle cells and intracystic tubules occurred in some cysts.