The E. coli dnaX and dnaZ gene products, essential for E. coli DNA replication, serve in chain elongation. Both genes, located at 10.4 min and previously cloned into a lambda vector and a ColE1 plasmid, were subcloned into pBR322 (pMK212). The coding region for the dnaX and dnaZ genes was localized to a 2.2-kb segment by deletion analysis of pMK212. The products of dnaX and dnaZ genes were identified as 78 kd and 52 kd polypeptides, respectively, by using maxicells bearing deletion clones of pMK212. Peptide mapping after limited proteolysis showed that the dnaZ gene product (52 kd) is a part of the dnaX gene product (78 kd), thus accounting for the coding capacity of the 2.2 kb region for both the dnaX and dnaZ genes. The dnaX gene product appears to be the tau subunit of DNA polymerase III holoenzyme; the dnaZ gene product is confirmed as the gamma subunit of the holoenzyme.