An insertion in rpoS, which encodes the general stress response sigma factor sigma 38, was isolated as an antimutator for 'stationary-phase' or 'adaptive' mutation. In the rpoS mutant strain the levels of error-prone DNA polymerase Pol IV were reduced. Pol IV is encoded by the dinB gene, and the amount of its transcript was also reduced in rpoS mutant cells. In wild-type cells, the levels of Pol IV increased in late stationary phase and stayed elevated for several days of continuous incubation, whereas in rpoS defective cells Pol IV was not induced and declined during prolonged incubation. Even in cells missing LexA, the repressor of dinB, maximum Pol IV expression required RpoS. These results suggest that induction of Pol IV is part of a cellular response to starvation and other stresses.