Potato virus M (PVM), a member of the carlavirus group was found to have a sedimentation coefficient (S20,w) of 157 S, a buoyant density in CsCl of 1.322 g.cm(-3), an extinction coefficient, at 260 nm, of 2.88 cm(2).mg(-1), and a nucleic acid content of 6.0%. The PVM genome is a polyadenylate-containing single-stranded RNA with a molecular weight (MW) of 2.5 x 10(6). Virus-specific, double-stranded RNA is consistently associated with PVM infection. PVM possesses a single capsid polypeptide with a MW of 35.7 x 10(3).