Electron microscopic studies of the spinal roots and the sciatic nerves of mutant hamsters with hindleg paralysis and of similar age control animals are reported. Fibrillary accumulations, some of which showed a crystalloid organization, were found in the inner loops of the myelin sheath. These changes were severe in the mutant but only mild in the controls. Moreover, the mutant alone showed demyelination with apparent sparing of the Schwann cell perikaryon and subsequent remyelination. A possible mechanism relating the fibrillary accumulations to the demyelination is discussed.