Computer-assisted comparative analysis of all available amino acid sequences of the capsid proteins of positive strand RNA plant viruses with helical capsids is described. Two distinct families of homologous proteins were delineated through statistically significant sequence similarities, one including the capsid proteins of rod-shaped viruses (tobamo-, tobra-, hordei-, and furoviruses) and the other those of filamentous viruses (poty-, bymo-, potex-, carla-, and closteroviruses). It was concluded that the capsid proteins of all rod-shaped viruses, on the one hand, and filamentous viruses, on the other hand, evolved from common ancestors. Analysis of residue conservation patterns in the capsid proteins of rod-shaped viruses revealed maintenance of the hydrophobic core and of the (putative) salt bridge between conserved Arg and Asp residues. Sequence comparisons within the filamentous virus family expanded the observations on the relationship between the capsid proteins of potex-, carla-, poty-, and bymoviruses. Grouping of the beet yellows closterovirus capsid protein sequence, recently determined in this laboratory (Agranovsky et al., J. Gen. Virol., 1991, 72, 15-23), with those of potex- and carlaviruses was demonstrated. The coat protein of another closterovirus, apple chlorotic leaf spot virus, appeared to constitute a distinct phylogenetic lineage. Despite the lack of significant overall similarity, comparison of the alignments of the capsid proteins of the two families suggested formation of analogous salt bridges.