The Drosophila melanogaster gene and cDNA which span the entire open reading frame for DNA polymerase alpha, were cloned, and their nucleotide sequences were determined. The gene consists of 6 exons separated by 5 short introns. The major transcription initiation site was localized 85 bp upstream from the initiation codon. The nucleotide sequence of the open reading frame revealed a polypeptide of 1,505 amino acid residues with a molecular weight of 170,796. The amino acid sequence of the polypeptide was 37% homologous with that of the catalytic subunit of human DNA polymerase alpha. This sequence contains six regions, the orders and amino acid sequences of which are highly conserved among a number of other viral and eukaryotic DNA polymerases. We found 7 amino acid residues in the region between the 639th and 758th positions, identical to those essential for the active site of Escherichia coli DNA polymerase I-associated 3'----5' exonuclease. Thus, the exonuclease activity may be associated with Drosophila DNA polymerase alpha. Levels of the DNA polymerase alpha mRNA were high in unfertilized eggs and early embryos, relatively high in adult female flies and second-instar larva, and low in bodies at other stages of development. This feature of the expression is similar to that of the proliferating cell nuclear antigen (an auxiliary protein of DNA polymerase delta) and seems to coincide with the proportions of proliferating cells in various developmental stages. As the half life of the mRNA for DNA polymerase alpha in cultured Drosophila Kc cells was 15 min, expression of the DNA polymerase alpha gene is probably strictly regulated at the step of transcription.