This is the first description of ‘ragged red fibers’ in extraocular muscles of myotonic dystrophy. Muscle biopsies were obtained from the biceps brachii and superior rectus of a 53-year-old man who had suffered from chronic progressive myotonia and muscular weakness in limbs, blepharoptosis, external ophthalmoparesis, myotonic cataract, and foveal pigmentary dystrophy. The extraocular as well as skeletal muscle cells showed centrally placed multiple nuclei with type 1 fiber predominance and increased intermyofibrillary connective tissue. About 20% of extraocular muscle fibers were identified as ‘ragged red fibers’ with the modified Gomori trichrome stain, most of which showed reduced activity in the cytochrome c oxidase reaction. An increased number of enlarged mitochondria were clustered beneath the sarcolemma and between the myofibrils, some of which contained irregularly or circularly arranged cristae and paracrystalline inclusions. Biochemical assessments showed in both skeletal and extraocular muscles decreased activity of mitochondrial enzymes, in particular the cytochrome c oxidase activity.