The Echigo-l strain of CJD was isolated by Mori and colleagues from a case of 33-years-old female with a panencephalopathic type of CJD. An incubation period following intracerebral inoculation of hamsters with l0 % cleared suspension of the Echigo-1-affected brain was approximately six months. We report here ultrastructural changes in the optic nerves. Vacuoles developed within myelinated axons: within axoplasm or within the myelin sheath and these were accompanied by exuberant reaction of macrophages and hypertrophied astrocytes. Axons underwent Wallerian degeneration and dystrophic neurites were also seen. Most important, we observed proliferation of inner mesaxons. Cross-sectional profiles of innumerable myelinated fibers contained membranous organelles which were continuous with the inner lamellae of the oligodendroglial cells. These unusual proliferations of inner mesaxons formed whorls and elaborated loops. In some axons, proliferation was so severe that loops of mesaxon filled the whole cross-section of the axon. Occasionally, we observed intrusion of the membranous tongue of the inner mesaxon into axoplasm.