Massive numbers of sarcocysts of a previously undescribed species of Sarcocystis were observed in the skeletal muscles throughout the body of an adult, female South American rattlesnake (Crotalus durissus terrificus). Examination of tissue sections by light microscopy demonstrated that sarcocysts were present in 20 to 40% of muscle fibers from 5 sampled locations. Sarcocysts were not present in cardiac muscle, smooth muscle, or other organs. Sarcocysts were 0.05–0.15 mm wide, had variable length depending on the viewed orientation and size of the muscle fiber, and had a sarcocyst wall less than 1-μm thick. Sarcocysts were subdivided by septa and had central degeneration in older sarcocysts. Host induced secondary encapsulation or an inflammatory response was not present. By transmission electron microscopy (TEM), the sarcocyst wall was Type I, with a parasitophorous membrane of approximately 100 nanometers in width arranged in an undulating pattern and intermittently folded inward in a branching pattern. The sarcocysts contained metrocytes in different stages of development and mature bradyzoites. The nucleic acid sequence from a section of the 18S small subunit rRNA gene was most closely related to S. mucosa that uses marsupials as intermediate hosts and has an unknown definitive host. This is apparently the third report of muscular Sarcocystis infection in snakes and is the first to describe the ultrastructure of the sarcocysts and use sequencing methods to aid in identification.