The identities of the DNA polymerases required for conversion of single-strand (ss) M13 DNA to double-strand (ds) M13 DNA were examined both in injected Xenopus laevis oocytes and in an oocyte nuclear extract. Inhibitors and antibodies specific to DNA polymerases alpha and beta were used. In nuclear extracts, inhibition by the antibody to polymerase beta could be reversed by purified polymerase beta. The polymerase beta inhibitors, dideoxythymidine triphosphate (ddTTP) and dideoxycytidine triphosphate (ddCTP), also blocked DNA synthesis and indicated that polymerase beta is involved in the conversion of ssDNA to dsDNA. These results also may have particular significance for emerging evidence of an ssDNA replication mode in eukaryotic cells.