A newly isolated blue-green algal virus, distinct in its morphology and host range, has been characterized physicochemically. The virus infects strains of the nitrogenfixing blue-green alga Nostoc muscorum. The virions have polyhedral heads, about 550 Å in diameter, and contractile tails which are 1100 Å in length. At least 19 proteins are found in mature virus particles. The nucleic acid is linear, double-stranded DNA, with a buoyant density of 1.696 g/cm3 and a sedimentation coefficient (s20,w) of 38.7 S. These values correspond to a G-C content of 37% and a molecular weight of 38 × 106, respectively.