A cucumovirus was isolated from M. sativa growing in the Barajas area near Madrid, Spain. After purification, the virus was identified as a strain of peanut stunt virus (PSV). In its host range, the virus (designated PSV-B) resembled previously characterized PSV strains (T, V and W) from the USA with only minor symptom differences in a few plant species. Serologically, PSV-B was indistinguishable from PSV-W and was closely related, but not identical, to PSV-V. There was a more distant serologic relationship between PSV-B and some strains of cucumber mosaic virus (CMV) and tomato aspermy virus. Although the sedimentation rate of PSV-B was identical to previously characterized cucumoviruses, in polyacrylamide gel electrophoresis PSV-B migrated with a different mobility than either PSV-V or PSV-W, although the difference from PSV-W was very small. It coelectrophoresed with CMV strain D. The virus is different from most other cucumoviruses in that it is very unstable in solution, where it degraded into nucleoprotein particles with bacilliform structures and lower sedimentation rates. These nucleoproteins contained the separate viral RNA components. Base ratios of the RNA of PSV-B, PSV-W and PSV-V revealed great similarity but were significantly different from that of CMV.