A new virus of the family Reoviridae has been isolated from adult chum salmon (Oncorhynchus keta) returning to the Tokushibetsu Hatchery, Hokkaido, Japan. The virus replicates in selected fish cell lines incubated between 10 and 20°C and produces a unique cytopathic effect. Electron micrographs reveal an icosahedral particle 75 nm in diameter composed of a double capsid. The outer capsid is selectively removed by α-chymotrypsin leaving a subviral particle with enhanced infectivity. The virus is resistant to pH 3 and chloroform. It is unstable at 56°C and does not hemagglutinate human type 0 erythrocytes. Infectivity is not reduced by antiserum to infectious pancreatic necrosis virus. Replication is not inhibited by 5-fluro2'-deoxyuridine. Acridine orange stain reveals typical reovirus-like cytoplasmic inclusions. The virus is not lethal for chum salmon fry, chinook salmon (O. tshawytscha) fry, or kokanee salmon (O. nerka) fry by injection. The virus replicates in the three species tested producing focal necrotic lesions in the liver of chum and chinook salmon fry.