Abstract
Three new groups (phleum mottle virus, southern bean mosaic virus and carnation mottle virus groups) are proposed for plant viruses which have small spherical particles sedimenting as a single component and each containing a single RNA species. The grouping is based on a range of characters including sedimentation coefficient, stabilization of the capsid, banding behaviour in Cs2SO4, protein subunit molecular weight and distribution of particles within the cell.