SUMMARYCocksfoot mottle virus (CFMV) was transmitted by manual inoculation of sap to cocksfoot (Dactylis glomerataL.), wheat, oats and barley, but not to nineteen other monocotyledonous and thirteen dicotyledonous plant species. The virus was also transmitted by cereal leaf beetles (Lema melanopaL.). Adult beetles infected plants more frequently than larvae, and remained infective for up to 2 weeks after they had fed on infected plants. Seed from infected cocksfoot and oat plants produced virus‐free seedlings.The infectivity of sap was lost during 10 min. at 65° C., and 2 weeks at 20° C., but survived many months at — 15° C.Purified virus preparations, made by various methods, contained numerous nearly spherical particles, about 30 mμ in diameter. In electron micrographs some of the particles were penetrated by negative stain though most appeared intact. However, all the particles migrated together in a centrifugal (sedimentation coefficient = 118 S) or electrophoretic field. The ultraviolet absorption spectrum, and the phosphorus and nitrogen contents of the virus preparations, were typical of a nucleoprotein containing about 25 % nucleic acid.Serological tests failed to show any relationship between CFMV and eleven other viruses with particles of similar shape a