Classical and ultrastructural neuropathology of prion diseases are generally well described. Here we report that alterations of myelinated fibres in hamsters infected either with polioencephalopathic strains of scrapie or panencephalopathic strains of CJD (Echigo-1) are virtually identical and differ only quantitatively. In contrast, mice infected with the panencephalopathic Fujisaki strain of CJD exhibited much more elaborate changes of myelinated fibres.