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The RNA genome (9346 nucleotides) of Oat necrotic mottle virus (ONMV) was cloned and sequenced. Complete genome comparisons indicated that ONMV, currently classified as a rymovirus, was most closely related (~73% nucleotide, ~79% amino acid identity) to the tritimovirus Wheat streak mosaic virus (WSMV). ONMV encoded a single polyprotein, with proteinase cleavage sites very similar to those ofWSMV. Pairwise comparison of ONMV andWSMV cistrons revealed that P3 was most conserved (~79% nucleotide, ~86% amino acid), whereas HC-Pro was most divergent (~67% nucleotide, ~69% amino acid). In contrast, the ONMV sequence was distantly related (~40% nt, ~26% amino acid identity) to that of the rymovirus Ryegrass mosaic virus, with highest sequence conservation noted within the NIb cistron (~47% nucleotide, ~41% amino acid identity). These results firmly establish that ONMV is not a rymovirus but is instead a distinct species of the genus Tritimovirus.