The 5′-linked ends of the nucleic acids of tobacco mosaic virus (TMV) and the RNA bacteriophages MS2 and f2 have been specifically labeled by means of periodate oxidation followed by reduction with tritiated borohydride. Labeled oligonucleotides characteristic of the end of each RNA were isolated through specific nuclease degradation and DEAE-cellulose chromatography, exploiting the non-phosphorylated character of the end fragment. Uniformly 32P-labeled viral RNA was used to determine the nucleotide composition of each isolated oligomer, and nucleotide sequences were examined by means of stepwise enzymatic degradation using spleen phosphodiesterase and by hydrolysis with specific nucleases. The 5′-linked end sequence of MS2 RNA is … GpUpUpApCpCpApCpCpCpA; the labeled oligonucleotide from ribonuclease T 1 digests of f2 RNA is chromatographically indistinguishable and expected to have the same sequence as that of MS2 RNA. The sequence of TMV RNA is confirmed to be … GpCpCpCpA.