The complete nucleotide sequence of the bottom component RNA (B RNA) of red clover mottle virus strain S has been determined. The sequence consists of 6033 nucleotides and contains a single long open reading frame sufficient to encode a protein of M(r) 210,258. The proteolytic processing sites within this protein have been deduced by comparison of its sequence with that of the B RNA-encoded protein of cowpea mosaic virus. Comparison of the amino acid sequences of the individual proteins confirms that the two viruses have a similar genome organization.