Cysts of a Sarcocystis species were found in large numbers in skeletal muscle of an otter (Lutra lutra) which was raised in Norway and died in captivity in Sweden. This is the first report of Sarcocystis infection in the otter. The sarcocysts were 0·3–2·3 mm long and 0·06–0·25 mm wide. As judged by light microscopy the sarcocyst walls were thin (<3 μm) with a serrated surface but without visible projections. By transmission electron microscopy, the sarcocyst wall measured 0·6–1·8 μm and had minute undulations covering the entire sarcocyst surface giving the wall a wavy appearance. Septa were indistinct. The sarcocysts contained few metrocytes and numerous bradyzoites. Sarcocysts were not found in 69 other otters subjected to necropsy in Sweden.