Among the viruses isolated in Japan, cucumber mosaic virus (CMV), soybean stunt virus (a soybean stunt strain of DMV), peanut stunt virus and chrysanthemum mild mottle virus belong to the cucumovirus group. All of these viruses contain four genomic RNA species (RNA!, RNA2, RNA3 and RNA4), and the three largest are necessary for infection. Pseudorecombinants (PRs) could be constructed either between these viruses or isolates of the same virus by exchange of RNA3. By comparing the properties among PRs and parents used, the determinant(s) of properties were Iocated in certain RNA species. Serological specificity and particle mobility were found to be determined by RNA3 of the cucumoviruses, while the symptoms on two plant species were determined by RNA1 +2. When fractionated RNA1 and RNA2 were exchanged between CMV isolates, symptoms on asparagus bean depended on RNA2.