The physicochemical and serological properties of a virus isolated from the bivalve mollusk T. tenuis were examined. The virus has a diameter of 59 nm, sediments at 430S in sucrose gradients and bands at a density of 1.32 g/ml in CsCl. The virus contains RNA with a MW about 2.8 .times. 106 as estimated by polyacrylamide gel electrophoresis but in sucrose gradients the RNA sediments at 14S. The virus RNA is resistant to RNase under conditions in which rRNA and the single stranded Mengo virus RNA are completely hydrolyzed. Two major polypeptides, MW 67 and 40 .times. 103, and 1 minor polypeptide, MW 110 .times. 103, are present in the virus particle. These properties are similar to those found for different serotypes of infectious pancreatic necrosis (IPN) virus. Although there was only a very low level of cross-neutralization between Tellina virus and IPN virus, there was some cross-reaction in immune EM tests and in immunofluorescence tests with infected [fish] tissue culture cells. This cross reaction, together with the close similarity in morphology and physicochemical properties, suggests that Tellina virus and IPN virus belong to the same virus group.