The relatedness of the genomes of satellite panicum mosaic virus (SPMV) and its helper virus, panicum mosaic virus (PMV), were investigated by nucleic acid hybridization. The results show that the satellite and helper virus RNAs have no appreciable homology or complementarity as assessed by hybridization with cDNA probes derived from the genomes of PMV and SPMV and with a probe complementary to the 3' terminus of SPMV RNA. The complete nucleotide sequence of SPMV RNA reveals that the genome is 826 nucleotides (nt) long. The ability to label SPMV RNA with polynucleotide kinase only after phosphatase treatment suggests that the 5' terminus is phosphorylated, but the extent of phosphorylation was not determined. The first open reading frame (ORF), encountered after an 88-nt 5'-untranslated region, encodes a 17,000 mol wt protein of a size and amino acid composition that are consistent with analysis of SPMV coat protein. An additional short ORF, located near the 3' end of the RNA, could encode a 6300 mol wt polypeptide. The minus strand also contains two ORFs that could potentially encode polypeptides of 7100 and 11,000 mol wt. No evidence is available to determine whether the second positive-strand ORF or the two minus-strand ORFs are expressed. The data presented here clearly show the SPMV RNA is distinct from the RNAs of other satellite viruses, in both size and nucleotide sequence. However, the 5'-untranslated portions of SPMV and satellite tobacco mosaic virus RNAs share some structural features that may be important in initiation of translation.