The complete 7410 nucleotide sequence of poliovirus type I genome was obtained from cloned cDNA. Double-stranded poliovirus cDNA was synthesized and inserted into the Pst I site of plasmid pBR322, and three clones were derived that together provided DNA copies of the entire poliovirus genome. Two of the clones contained inserts of 2.5 and 6.5 kilobases and represented all but the 5' 115 bases of poliovirus RNA. A third clone was generated from primer-extended DNA and contained sequences from the 5' end of the viral RNA. An open reading frame that was identified in the nucleotide sequence starting 743 bases from the 5' end of the RNA and extending to a termination codon 71 bases from the 3' end contained known poliovirus polypeptide sequence.