Nearly full-sized double-stranded cDNA was prepared from virion RNA of poliovirus Sabin 1 (LSc, 2ab) strain using reverse transcriptase. The double-stranded cDNa was cleaved at four and two sites by the restriction endonucleases Bam HI and Hind III, respectively. Based on the cleavage patterns of double-stranded cDNA into segments of various lengths, each of which had the sequence corresponding to that of the 3'-end of the viral genome, the location of each restriction fragment was determined. The cDNA fragments were cloned with pBR322 as a vector and some of their nucleotide sequences were determined. The DNA sequence indicated that the restriction map obtained was consistent with the known arrangement of viral RNA fragments (1-3). Comparison of the nucleotide sequences of the cloned Hind III (460 bases) and Pst I (434 bases) fragments with those of the corresponding region of Mahoney type 1 genome (3) revealed 12 point-mutation sites.