We report the ultrastructural appearances of myelinated fibres of the optic nerves from mice infected with the Fujisaki strain of Gerstmann-Sträussler-Scheinker (GSS) disease and from Echigo-1 strain of Creutzfeldt-Jakob disease (CJD). Optic nerves from CJD- and GSS-infected rodents showed severe pathological changes. Theses changes were qualitatively indistinguishable from each other but were more robust in the Fujisaki GSS model than in the hamsters inoculated with Echigo-1. The most characteristic finding was the distension with the attenuation of the myelin sheath forming a vacuole while shrunken axon was attached to the innermost layer of the myelin. With high power electron microscopy, we could observe the splitting of the myelin lamellae at the major dense or intraperiod lines to form complex openings that extended to line the vacuole. This finding suggests that myelin lamellae participate in the formation of vacuoles.