A 12-year-old girl with proximal muscle weakness and delayed psychomotor development from early infancy is presented. She had a myopathic face, high-arched palate, nasal tone vocalization, positive Gowers' sign, waddling gait and decreased deep tendon reflexes. Her IQ was 40 (PIQ = 39, VIQ = 51). The serum creatine kinase level and peripheral nerve conduction velocity as well as electrocardiogram were normal. The electromyogram showed myopathic changes. Magnetic resonance imaging (MRI) of thigh muscles revealed fatty infiltration of all muscle groups, the hypertrophic biceps femoris and semimembranous muscles being spared. A biopsy specimen from the left biceps brachii muscle revealed small caliber fibers, increased variability of fiber size and uniformity of type 1 fibers (greater than 99%). This case was diagnosed as having congenital nonprogressive myopathy with uniform type 1 fibers, and had a non-deteriorating clinical course as in most congenital nonprogressive myopathy cases.