The sequences of adaptive reversions of a lac frameshift mutation in Escherichia coli resemble DNA polymerase errors, and the adaptive reversions decrease in strains with an antimutator DNA polymerase III (PolIII) allele. The latter finding could imply that DNA PolIII itself makes adaptive mutations. Alternatively, normal DNA PolIII errors could saturate post-synthesis mismatch repair during adaptive mutation. If so, the antimutator strain would produce fewer adaptive mutations because it possesses greater capacity for mismatch repair which could correct errors made by a polymerase other than DNA PolIII. Mismatch repair capacity is limited specifically during adaptive mutation, necessitating a test of this indirect model. This indirect model is ruled out here by the observation that the antimutator PolIII allele decreases adaptive mutation even in mismatch repair-defective cells. This supports a direct role for DNA PolIII in recombination-dependent adaptive mutation.