We are in the process of sequencing the poliovirus genome by an adaptation of Sanger's dideoxy method. This involves reverse transcription of virion RNA, followed by E. coli Pol I catalyzed DNA synthesis in the presence of cDNA, dNTPs, ddNTPs, and [5′-32P]labeled primers (RNase T1 or RNase A resistant oligonucleotides of virion RNA). So far, we have sequenced segments of the RNA totalling 7,000 nucleotides. A sequence of 1,060 bases from the 3′ end reveals that at least 562 nucleotides preceding the poly(A) remain untranslated. A base sequence in the 3′-terminal half of the genome corresponding to known amino acid sequences of the genome-linked protein (VPg) has been detected. These data suggest that VPg is part of the viral polypeptide NCVPlb. Thus, VPg and the viral RNA polymerase NCVP4 originate from the same polypeptide precursor. VPg is, at most, 27 amino acids long (max. mol. wt. 3, 296).