Several atypical features of myelination of the peripheral nervous system are reported in common peroneal nerve of dystrophic mice (129 Re J dy/dy): (i) central nervous system-like contact between myelin sheaths of adjacent nerve fibres; (ii) nodes and internodes of myelinated fibres enwrapped with cytoplasmic processes of Schwann cells from adjacent nerve fibres; (iii) Schwann cells of adjacent nerve fibres co-operating in formation of a single myelin sheath; and (iv) a single Schwann cell myelinating two separate axons. In view of the presence of similar features of myelination in the central nervous system, where the myelin producing cells lack basement membrane, we suggest that in the dystrophic peripheral nerves the development of these features can be attributed to the partial deficiency of the Schwann cell basement membrane. Two types of widened nodes of Ranvier are also identified: (i) nodes with paranodal damage; and (ii) nodes without paranodal damage. In addition, abnormal features of myelination are described which are likely to represent altered Schwann cell/axon relationships during demyelination and remyelination and/or decreased myelinating ability of Schwann cells. We interpret these findings as indicating a metabolic disorder of Schwann cells. They provide an experimental model for the investigation of factors involved in the origin and maintenance of the structural organization of peripheral nerve.