Histopathological lesions of the central nervous system are described hatchery-reared larval seabass (Dicentrarchus labrax)during mass mortalities. Lesions are consistent with an extensive vacuolation of the brain and retina, associated with intracytoplasmic inclusions, containing RNA as demonstrated with acridine-orange epifluorescence. Electron microscope observations revealed numerous cytoplasmic, non-enveloped particles about 22-23 nm in diameter. Isopycnic centrifugation in a CsCl gradient produced two bands of virus particles showing buoyant densities of 1.30 to 1.34 g/cm super(3), respectively. Negatively stained preparations exhibited full and empty icosahedral particles about 26 nm in diameter from side to side and 29 nm from point to point. By their characteristics, these particles are closely related to the Picornaviridae family.