The incorporation of m6dATP by T4 DNA polymerase has been investigated. Unlike Escherichia coli DNA polymerase I (Engel, J.D., and von Hippel, P.H. (1978) J. Biol. Chem, 253, 935-939), the T4 enzyme discriminates at the insertion step against the methylated triphosphate as compared to the normal substrate (dATP). The apparent Km values measured in two ways agree with the overall 7-fold discrimination measured in double label experiments. The apparent Vmax values measured for net DNA synthesis are the same, while those measured for nucleotide turnover show that the rate for m6dATP is 2-fold greater than for dATP itself. The T4 enzyme results are consistent with the generally held theory that fidelity at the insertion step of DNA polymerization is determined by the relative free energies of primer-enzyme-triphosphate ternary complexes formed by competing, alternative substrate dNTPs. These results are also consistent with the view that these free energies chiefly depend on formation of satisfactory hydrogen bonds between the bases of the template and triphosphate.