A poliovirus-specific polyuridylic acid [poly(U)] polymerase that copies a polyadenylic acid template complexed to an oligouridylic acid primer was isolated from the membrane fraction of infected HeLa cells and was found to sediment at 4 to 5S on a linear 5 to 20% glycerol gradient. When the poly(U) polymerase was isolated from cells labeled with [(35)S]methionine and was analyzed by glycerol gradient centrifugation and polyacrylamide gel electrophoresis, the position of only one viral protein was found to correlate with the location of enzyme activity. This protein had an apparent molecular weight of 62,500 based on its electrophoretic mobility relative to that of several molecular weight standards and was designated p63. When the poly(U) polymerase was isolated from the soluble fraction of a cytoplasmic extract, the activity was found to sediment at about 7S. In this case, however, both p63 and NCVP2 (77,000-dalton precursor of p63) cosedimented with the 7S activity peak. When the 7S polymerase activity was purified by phosphocellulose chromatography, both p63 and NCVP2 were found to co-chromatograph with poly(U) polymerase activity. The poliovirus replicase complexed with its endogenous RNA template was isolated from infected cells labeled with [(35)S]methionine and was centrifuged through a linear 15 to 30% glycerol gradient. The major viral polypeptide component in a 26S peak of replicase activity was p63, but small amounts of other poliovirus proteins were also present. When the replicase-template complex was treated with RNase T1 before centrifugation, a single peak of activity was found that sedimented at 20S and contained only labeled p63. Thus, p63 was found to be the only viral polypeptide in the replicase bound to its endogenous RNA template, and appears to be active as a poly(U) polymerase either as a monomer protein or as a 7S complex.