Beet mosaic virus (BtMV) was identified almost five decades ago but has not been fully characterized at the molecular level. In this study, we have determined for the first time the complete nucleotide sequence of BtMV genomic RNA and have developed a specific molecular means for its diagnosis. The viral genome of BtMV comprises 9591 nucleotides, excluding the 3' terminal poly (A) sequence, and contains a single open reading frame (ORF) that begins at nt 166 and terminates at nt 9423, encoding a single polyprotein of 3086 amino acid residues. A 3' untranslated region of 168 nucleotides follows the ORF. The deduced genome organization is typical for a member of the family Potyviridae and includes 10 proteins: P1, HC-Pro, P3, 6K1, CI, 6K2, NIa-VPg, NIa-Pro, NIb and coat protein (CP). Nine putative protease cleavage sites were predicted computationally and by analogy with genome arrangements of other potyviruses. Conserved sequence motifs of homologous proteins of other potyviruses were found in corresponding positions of BtMV. BtMV is a distinct species of the genus Potyvirus with the most closely related species being Peanut mottle virus ( approximately 55% amino acid identity). Based on the nucleotide sequence obtained, we have developed a virus-specific RT-PCR assay for accurate diagnosis and differentiation of BtMV.