A 20-month-old boy--offspring of consanguinous parents, whose mother presumably had subclinical myopathy--presented with clinical signs of congenital non-progressive myopathy, neurogenic-myogenic electromyographic findings and normal motor conduction velocity. Biopsy of quadriceps muscle showed fiber-type disproportion with hypotrophic type 1, hypertrophic 2A and absent 2B fibers. Subsarcolemmal segmental foci of abnormally, in part regularly arranged bundles of mostly thin myofilaments were found in 13% of hypotrophic type 1 fibers. Rods were seen in only 1 fiber out of 20 tissue blocks. Reexamination 6 years later revealed slightly increased muscle force, myopathic EMG pattern and borderline motor and sensory nerve conduction velocities. Biopsy specimen from deltoid muscle consisted of untypable fibers of varying diameters with jagged Z-lines and increased variability of myofibrillar diameters. Multiple rods were present in 1% of the fibers, the formerly seen segmental foci in 0.1% only. Several intramuscular nerves were normal. The case contributes some new features to the spectrum of congenital myopathies of the nemaline type and suggests different stages of arrested maturation of type 1 fibers at least in this particular case.