Ribgrass mosaic virus (RMV) is a member of the tobamovirus group of plant viruses. The structure has been determined at 2.9 A resolution by fiber diffraction methods, and refined by molecular dynamics methods to an R-factor of 0.095. The carboxyl-carboxylate interactions that drive disassembly in tobamoviruses are present in RMV, but are very different from those in other tobamoviruses. RMV has some of the structural features of a subgroup I tobamovirus, a smaller number from subgroup II, and a number that appear to be unique to the RMV cluster of viruses. The structural studies confirm the evolutionary and taxonomic separation of the RMV cluster from both subgroup I and subgroup II tobamoviruses.