Abstract A new virus infecting the unicellular blue-green alga, Synechococcus strain NRC-1, has been isolated. The virus, named S-1, possesses a head 500 Å in diameter and a narrow, noncontraotile tail 1400 Å in length. The sedimentation coefficient of whole virus particles is 353 S and the buoyant density is 1.501 g/cm 3. The major structural proteins have molecular weights of 10,000, 11,000, and 39,000 daltons. S-1 DNA has a contour length of 13.3 ± 0.3 μm and a S 20, w of 30.8 ± 1.5 S. These values imply a molecular weight of 23–26 × 10 6 daltons. The buoyant density in CsCl of S-1 DNA is 1.729 g/cm 3, suggesting a GC content of 70%. A slightly higher value, 74%, is derived from the thermal denaturation temperature of S-1 DNA.