The nucleotide sequence of the Club Lake isolate of turnip yellow mosaic virus (TYMV-CL) genomic RNA has been determined. The genome is 6319 nucleotide residues in length and has three major open reading frames (ORFs), two of which overlap. The smallest ORF is proximal to the 3' terminus and encodes the virion protein gene, which has 98% sequence similarity with the virion protein gene reported for the type strain of TYMV. The largest ORF is from nucleotide residues 96 to 5630, and encodes a protein some parts of which show sequence similarities to the possible RNA replicases and nucleotide binding proteins of other viruses. The third ORF is from nucleotide residues 89 to 1975 and overlaps the 5' end of the largest ORF in a manner similar to that found in several animal viral genomes. The function of the protein encoded by this ORF is unknown. The genomes of tymoviruses have, characteristically, an unusually large cytosine content and small guanosine content. This compositional bias is mirrored in the codon and dinucleotide frequencies of the TYMV-CL genome, but is only partially reflected in the amino acid sequences encoded by the genome.