The transmissible spongiform encephalopathies are a group of subacute progressive degenerative diseases of the nervous system which are always fatal in their outcome. These diseases appear to be caused by the abnormal isoform of the precursor protein of amyloid designated prion protein. The normal isoform has been identified in the tissues of all mammalian species thus far tested as well as in Drosophila. We report the presence of this protein for the first time in the brains of fish.