Dorsal and ventral spinal roots at cervical, thoracic, lumbar and sacral levels in dystrophic, dy/dy, mice of both 129/ReJ and C57Bl/6J phenotypes showed a complete lack of Schwann cell sheaths of any sort around the majority of their nerve fibres. This condition, termed amyelination, is more extensive (up to 1-5 cm in length) in the longer lumbar and sacral roots than in the shorter roots or in the proximal regions of the sciatic nerve which are also affected to some extent. Amyelination does not appear to be a consequence of myelin or Schwann cell degeneration, as debris is uncommon. Heterozygous carriers are not affected in any obvious way. Myelinated fibres, with Schwann cells of peripheral origin, occur immediately adjacent to the spinal cord in both dorsal and ventral roots, while in dorsal roots unmyelinated fibres also occur, as in normal animals. Amyelinated fibres begin to appear a few internodal lengths away from the cord and are present until near, or within, the dorsal root ganglion, where they become myelinated again. The portion of an axon which has no myelin begins at a normal appearing paranodal region (termed a half node of Ranvier) at the end of a myelin internode. Resumption of myelination likewise begins at a half node. A few myelinated axons may be seen in any given cross section of a root, but as a rule a given myelinated fibre does not remain myelinated throughout the whole length of the root. It is suggested that the nerve lesions develop where the nerves are lengthening rapidly as the animal grows and changes its shape. How these nerve changes release to those in muscle is conjectural.