DNA polymerases change their specificity for nucleotide substrates with each catalytic cycle, while achieving error frequencies in the range of 10(-5) to 10(-6). Here we present a 2.2 Angstrom crystal structure of the replicative DNA polymerase from bacteriophage T7 complexed with a primer-template and a nucleoside triphosphate in the polymerase active site. The structure illustrates how nucleotides are selected in a template-directed manner, and provides a structural basis for a metal-assisted mechanism of phosphoryl transfer by a large group of related polymerases.