A phylogenetic analysis of the five major families of DNA polymerase is presented. Viral and plasmid sequences are included in this compilation along with cellular enzymes. The classification by Ito and Braithwaite (Ito and Braithwaite 1991) of the A, B, C, D, and X families has been extended to accommodate the "Y family" of DNA polymerases that are related to the eukaryotic RAD30 and the bacterial UmuC gene products. After analysis, our data suggest that no DNA polymerase family was universally conserved among the three biological domains and no simple evolutionary scenario could explain that observation. Furthermore, viruses and plasmids carry a remarkably diverse set of DNA polymerase genes, suggesting that lateral gene transfer is frequent and includes non-orthologous gene displacements between cells and viruses. The relationships between viral and host genes appear very complex. We propose that the gamma DNA polymerase of the mitochondrion replication apparatus is of phage origin and that this gene replaced the one in the bacterial ancestor. Often there was no obvious relation between the viral and the host DNA polymerase, but an interesting exception concerned the family B enzymes: in which ancient gene exchange can be detected between the viruses and their hosts. Additional evidence for horizontal gene transfers between cells and viruses comes from an analysis of the small damage-inducible DNA polymerases. Taken together, these findings suggest a complex evolutionary history of the DNA replication apparatus that involved significant exchanges between viruses, plasmids, and their hosts.