Cucumoviruses derive their name from the type member cucumber mosaic virus (CMV). CMV has been isolated from no less than 775 species of 174 genera in 52 families of flowering plants and can be transmitted by more than 60 species of aphids. A number of viruses other than CMV have been described as members or likely members of the group. The Cucumovirus capsid is built from a single species of polypeptide. The importance of protein-RNA interactions for the integrity of the Cucumovirus particle can be seen from its disintegration as the encapsidated RNA is digested with ribonuclease. A number of cytopathic effects, other than the accumulation of virus particles, have been observed in plant cells infected by Cucumoviruses. Pseudorecombinants have been constructed in vitro from different Cucumovirus isolates to determine the distribution of genes on the three segments of their genomes.