Nalidixic acid-resistant mutants of Campylobacter jejuni and C. coli as well as "C. laridis" strains showed cross-resistance to another DNA gyrase subunit A inhibitor, enoxacin (MIC, 32 micrograms/ml), whereas C. fetus subsp. fetus, C. fetus subsp. venerealis, and "C. hyointestinalis" strains were all susceptible to enoxacin (MIC, less than or equal to 2 micrograms/ml). All Campylobacter species were resistant to novobiocin (MIC, 32 to 512 micrograms/ml), but most strains were susceptible to the other DNA gyrase subunit B inhibitors coumermycin A1 and clorobiocin.