Complementary DNAs encompassing the coat protein coding and adjacent regions of Agropyron mosaic virus (AgMV) and Hordeum mosaic virus (HoMV) were cloned and sequenced. Comparison with other sequenced potyviruses indicated that each clone contained the 3'-non-coding region (3'NCR), the coat protein (CP) gene and part of the nuclear inclusion protein (NIb) gene. Nucleotide and amino acid sequence comparisons of the 3'-terminal regions of these and other rymoviruses indicate that distinct groups exist. Ryegrass mosaic virus (RGMV) strains share sequence similarity with AgMV and HoMV. Wheat streak mosaic virus (WSMV) and brome streak mosaic virus (BrSMV) form a separate group, sharing limited sequence similarity with the other rymoviruses. It is proposed that subgroups occur within the Rymovirus genus, depending on the vector species involved in transmission and on sequences.