Diphtheria toxin was injected into the electric organ of the gymnotid fish, Sternarchus albifrons. After 10 days, there was extensive demeylination of electrocyte fibers in the area of injection. Electron microscopy showed that paranodal loops of myelin do not separately cleanly from the axon, and remnants of the myelin loops may persist after demyelination of the internodes is nearly complete. The dense cytoplasmic undercoating of the nodal axolemma may disappear before the paranodal junctions are completely gone. Observations of demyelination of internodes between the elaborate, inexcitable nodes suggest that the presence of myelin may not be necessary for the maintenance of structural differentiation of this region of the axolemma. Use of diphtheria toxin to demyelinate Sternarchus electrocytes may provide a useful system for experimental neuropathological studies.