Three distinct sequence groups were found among partial nucleotide sequences of 38 isolates of beet western yellows virus (BWYV) and beet mild yellowing virus (BMYV) from Europe, Iran and the USA. The first group contains both sugar beet and oilseed rape specific isolates, and the differentiating characteristic linked to this host range specificity are 2 single base pair changes in a 1,200 nucleotide region of the genome. It is proposed that the European BWYV strains that can be transferred at low frequency between rape and sugar beet belong to this group. Also belonging to this group are the published BWYV sequences of Veidt et al. and of the California BWYV-ST9 isolate. The second group contains mostly rape-derived isolates which have an intergenic region highly distinct from that of group-1 isolates but similar polymerase and coat protein regions. It is proposed that the rape-specific BWYV isolates which cannot be transmitted to sugar beet belong to this group. The third group contains mostly beet-specific isolates from Southern Europe and Iran, and may be adapted to the Mediterranean climate and flora. It is distinct from groups 1 and 2 in all three genome regions investigated and its polymerase and intergenic regions are as much related to those of potato leafroll virus (PLRV) and curcurbit aphid borne yellows virus (CABYV) as they are to those of group-1 and group-2. On the basis of sequence similarities and established nomenclature it is proposed to use BWYV for groups 1 and 2 (BWYV-1 and BWYV-2 respectively) and to use BMYV for group-3 isolates, which are distinct enough from the other two groups to merit a separate nomenclature.