The nepoviruses (Harrison et al. 1971) are a group of nematode-transmitted plant viruses that have isometric particles with slightly angular outlines and diameters of 28 to 30 nm. Purified preparations of these viruses typically contain particles of three types, differing in RNA content (Stace-Smith, Reichmann & Wright, 1965; Diener & Schneider, 1966). The particle structure of nepoviruses has been little studied but, using evidence obtained by electron microscopy of negatively stained preparations, it was proposed that the protein shells of two nepoviruses, tobacco ringspot (Chambers, Francki & Randles, 1965) and arabis mosaic (Agrawal, 1967), are composed of 42 morphological subunits. This work did not, however, take into account all other possibilities and our results, described below, show that the particles cannot be built of 42 morphological subunits.