Most replicases are multi-subunit complexes. DNA polymerase epsilon from Saccharomyces cerevisiae is composed of four subunits: Pol2p, Dpb2p, Dpb3p, and Dpb4p. Pol2p and Dpb2p are essential. To investigate a possible role for the Dpb2p subunit in maintaining the fidelity of DNA replication, we isolated temperature-sensitive mutants in the DPB2 gene. Several of the newly isolated dpb2 alleles are strong mutators, exhibiting mutation rates equivalent to pol2 mutants defective in the 3' --> 5' proofreading exonuclease (pol2-4) or to mutants defective in mismatch repair (msh6). The dpb2 pol2-4 and dpb2 msh6 double mutants show a synergistic increase in mutation rate, indicating that the mutations arising in the dpb2 mutants are due to DNA replication errors normally corrected by mismatch repair. The dpb2 mutations decrease the affinity of Dpb2p for the Pol2p subunit as measured by two-hybrid analysis, providing a possible mechanistic explanation for the loss of high-fidelity synthesis. Our results show that DNA polymerase subunits other than those housing the DNA polymerase and 3' --> 5' exonuclease are essential in controlling the level of spontaneous mutagenesis and genetic stability in yeast cells.