In addition to the intracytoplasmic rods in approximately 1/4 muscle fibers, there were a large number of fibers with fetal muscle characteristics in a female infant who had severe muscle weakness and hypotonia, and failure to thrive since birth. A histochemical examination disclosed abnormal distribution in muscle fiber types including remarkable type 1 fiber predominance and increased number of type 2C fibers (11.4 %). Scattered throughout were fibers consisting of multiple myocytes enclosed in a single basement membrane, and small-calibered fibers containing abundant intermediate (skeletin) filaments and dispersed microtubules. Both were histologically identical to premature fibers found in the fetal muscle. The existence of an increased number of satellite cells as compared with age-matched controls was also suggestive of delayed or arrested muscle fiber maturation. A certain impaired neural influence upon the developing muscle is probably responsible for producing abnormal fiber type distribution and immature small-calibered fibers which account for small muscle bulk and muscle weakness in nemaline myopathy.