The complete nucleotide sequence of a UK isolate of oat mosaic virus (OMV) was determined. RNA1 was 7550 nt long with one large open reading frame potentially encoding a polyprotein of 262.8 kDa with features typical of bymoviruses. RNA2 was 2284 nt in length, substantially smaller than those of other bymoviruses sequenced. It appeared that most of the P2 region had been deleted during repeated mechanical transmission of the isolate. The 3'-UTR of RNA2 was very long (> 1.25 kb) and proved to have a 532 nt slightly overlapping repeat. Phylogenetic analysis confirmed that OMV is an independent member of the genus Bymovirus.