DNA polymerase beta is a small monomeric polymerase that participates in base excision repair and meiosis [Sobol, R., et al. (1996) Nature 379, 183-186; Plug, A., et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 1327-1331]. A DNA polymerase beta mutator mutant, F272L, was identified by an in vivo genetic screen [Washington, S., et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 1321-1326]. Residue 272 is located within the deoxynucleoside triphosphate (dNTP) binding pocket of DNA polymerase beta according to the known DNA polymerase beta crystal structures [Pelletier, H., et al. (1994) Science 264, 1891-1893; Sawaya, M., et al. (1997) Biochemistry 36, 11205-11215]. The F272L mutant produces errors at a frequency 10-fold higher than that of wild type in vivo and in the in vitro HSV-tk gap-filling assay. F272L shows an increase in the frequency of both base substitution mutations and frameshift mutations. Single-enzyme turnover studies of misincorporation by wild type and F272L DNA polymerase beta demonstrate that there is a 4-fold decrease in fidelity of the mutant as compared to that of the wild type enzyme for a G:A mismatch. The decreased fidelity is due primarily to decreased discrimination between the correct and incorrect dNTP during ground-state binding. These results suggest that the phenylalanine 272 residue is critical for maintaining fidelity during the binding of the dNTP.