A 23-year-old man had muscular weakness and wasting starting in childhood. Biopsy material from quadriceps femoris showed groups of small fibers, with a selective decrease in diameter of type I fibers. Two additional biopsy specimens from deltoid showed numerous nemaline bodies in small type I fibers. Electron microscopically, two mutually perpendicular sets of striations were visualized in both cross and longitudinal sections of these bodies. These bodies seemed to consist of crystals whose sides measured 105 Angstroms and length, 180 or 370 A. The term "nemaline myopathy" was avoided because it is uncertain whether the disorder was actually myopathic.