Multifocal myositis was diagnosed in a 7-year-old Quarter Horse gelding on the basis of history and findings on physical examination, serum biochemical analysis, electromyography, and microscopic examination of frozen sections of muscle biopsy specimens. Histologic examination of the muscle specimen revealed multifocal accumulations of histiocytes, lymphocytes, and plasma cells, with attendant myofiber degeneration and necrosis. Parasitic cysts with morphologic characteristics of Sarcocystis sp were found in regions of myocyte degeneration and necrosis, and in regions of normal muscle. Based on a tentative diagnosis of Sarcocystis sp-induced myositis, the horse was treated with trimethoprim/sulfamethoxazole and pyrimethamine for 28 days, phenylbutazone for 5 days, and paddock rest for 30 days. At the end of treatment, the horse had gained 35 kg, its appetite had returned to normal, and muscle mass was returning to normal. Sarcocystis fayeri is the only Sarcocystis sp reported in equine muscle in the United States and is rarely associated with acute myositis or muscle atrophy. The development of clinical signs in this horse could have been the result of an underlying immunosuppression or infection with a particularly pathogenic strain or large infective dose of S fayeri.