Muscular sarcocystosis is a rare infection in dogs. Clinical myositis associated with an unidentified species of Sarcocystis was diagnosed in an adult dog from Canada. There was granulomatous myositis associated with numerous immature sarcocysts in a muscle biopsy obtained from the dog. The sarcocysts were up to 550 μm long and up to 45 μm wide. The sarcocyst wall was approximately 1 μm thick and contained short, stubby, villar protrusions that lacked microtubules. This is the first report on clinical muscular sarcocystosis in a dog.