Cocksfoot mottle virus is a plant virus that belongs to the genus Sobemovirus. The structure of the virus has been determined at 2.7 A resolution. The icosahedral capsid has T = 3 quasisymmetry and 180 copies of the coat protein. Except for a couple of stacked bases, the viral RNA is not visible in the electron density map. The coat protein has a jelly-roll beta-sandwich fold and its conformation is very similar to that of other sobemoviruses and tobacco necrosis virus. The N-terminal arm of one of the three quasiequivalent subunits is partly ordered and follows the same path in the capsid as the arm in rice yellow mottle virus, another sobemovirus. In other sobemoviruses, the ordered arm follows a different path, but in both cases the arms from three subunits meet and form a similar structure at a threefold axis. A comparison of the structures and sequences of viruses in this family shows that the only conserved parts of the protein-protein interfaces are those that form binding sites for calcium ions. Still, the relative orientations and position of the subunits are maintained.