A hitherto undescribed virus, Molinia streak virus (MS V), has been found in plants of Molinia coerulea (L.) Moench. It is readily sap-transmitted, but only to graminaceous plants, and has a thermal inactivation point of about 85°. Its particles are isometric, have a 28-nm diameter, sediment at 112S and contain about 22% RNA with a base composition of A 28.2, G 23.8, C 25.5, and U 22.5. The single species of protein in the virion has a molecular weight of 26,000 daltons. MSV is distantly related serologically to cocksfoot mild mosaic and phleum mottle viruses