E. coli AB1157 cultures incubated for ten days on solid media containing 4 or 8 μg mL−1 nalidixic acid (NA) (one or two times the MIC respectively, with the latter concentration being bactericidal) showed an increased apparent mutation frequency to NA resistance. The presence of the mutator plasmids R46 and R446b further increased the number of NA-resistant mutants recovered. Growth of E. coli AB1157 in liquid medium containing 2 or 3 μg mL−1 NA (0.5 and 0.75 × MIC) for ten days increased the number of mutants resistant to 16 μg mL−1 NA (4 × MIC), compared to control cultures grown in drug-free broth. Again, R46 and R446b greatly enhanced this effect. As a control, no increase in reversion of the unselected allele, thr-1 was detected during ten days growth in broth containing 0, 2 or 3 μg mL−1 NA. These results suggest that the presence of NA might induce mutational resistance to itself via error-prone repair. Strains carrying defective recA13, umuC36, lexA3 or recB21 alleles, deficient in error-prone repair, displayed a reduction in the numbers of NA resistant mutants recovered after four days incubation in broth containing 0.75 × MIC NA. The presence of a defective uvrA gene led to an increase in the number of NA-resistant mutants recovered, suggesting enhanced error-prone repair of NA-induced DNA lesions in the absence of error-free excision repair. Clinically important quinolone-resistance mutations usually occur at position Ser-83 in the gyrA gene, within the highly conserved quinolone resistance-determining region (QRDR). Sequence analysis of the QRDR of four quinolone-resistant AB1157 clones revealed only single changes. Three had mutated at the Ser-83 position, and one showed variation at position Asp-87. It would therefore appear that NA-induced mutation to NA resistance selected in vitro may well be clinically significant.