A case of sarcosporidiosis in a 29 year-old woman is presented. The clinical picture showed a definitive myopathic pattern with symmetrical proximal weakness of shoulder and pelvic girdle and also weakness on neck flexors; the facial muscles were intact and she denied dysphagia. The histological findings after a muscle biopsy obtained from left deltoid muscle showed many round cysts within the muscle fibres without any surrounding inflammatory reaction. The cysts diameters varied from 30 to 500 micra and the lenght from 300 micra to 3 milimeter (Miescher's tube). The radially striated membrane could be seen in some cysts but there was some doubt because it could represent the remaining of the peripheral muscle fibre. We could not find spores inside the cysts. Besides the largest cysts there were muscle fibre hypertrophy. The authors concluded on the diagnosis of sarcosporidiosis because of the presence, diameter and lenght of cysts; some remarks are made on the only other parasite that could be confused with Sarcocystis sp: Toxoplasma gondii.