T5 DNA polymerase (T5Pol), an essential enzyme for bacteriophage T5 DNA replication, is unusual because of its high processivity and strand-displacing ability. These two properties in a single polypeptide make T5Pol an ideal candidate for structural and functional analysis. Therefore, the structural gene encoding the DNA polymerase of bacteriophage T5 (T5pol) has been cloned and overexpressed in Escherichia coli. Elimination of sequences upstream from the 5' end of the T5pol by exonuclease III digestion was necessary to obtain stable clones containing a full-length structural gene. Determination of the nucleotide (nt) sequence of the region deleted during clone construction revealed the presence of a promoter sequence having extensive homology with known T5 phage 'early' promoters. By primer extension of mRNA isolated from T5 phage-infected cells, two successive G residues located 6 and 7 nt downstream from the -10 region of this promoter were identified as the initiating nt at the 5' end of T5pol mRNA. T5Pol produced in E. coli from the cloned gene under control of a tac or phage lambda pL promoter represented as much as 40% of total cell protein. The majority of the T5Pol present in extracts of E. coli was insoluble. The amount of active enzyme present was estimated to be a maximum of tenfold higher than that found in extracts of T5 phage-infected cells.