Sarcocystis cymruensis was initially identified in skeletal muscles of 22 (11.6%) of 189 wild rats (Rattus spp.) captured in 2008 in Anning and Kunming, Peoples Republic of China. Sarcocyst walls were thin (<1 µm) and smooth. Ultrastructurally, the parasitophorous vacuolar membrane had small, osmiophilic knob-like invaginations covered with numerous vesicle-like invaginations toward the interior of the cyst. Domestic cats (Felis catus) fed sarcocysts shed sporocysts measuring 10.3 (9.8–11.0) × 7.6 (7.2–9.5) µm with a prepatent period of 6 to 8 days. Sarcocysts were infective orally to Norway rats, and oocysts and sporocysts developed in the lamina propria of the small intestine of rats fed sarcocysts. Thus, rats were both intermediate and definitive hosts for S. cymruensis.