DNA gyrase, the bacterial enzyme that supercoils DNA, is trapped on chromosomal DNA by the 4-quinolone compounds, as drug-gyrase complexes that contain DNA breaks. Examination of chromosomal DNA extracted from Escherichia coli indicated that bacteriostatic concentrations of oxolinic acid trap gyrase and block DNA synthesis without releasing broken DNA from gyrase-DNA complexes. Release, detected as free rotation of DNA in the presence of an intercalating dye, occurred only at high, bactericidal oxolinic acid concentrations. Release of DNA breaks and cell death were both blocked by chloramphenicol, an inhibitor of protein synthesis, suggesting that synthesis of additional protein activity is required to free the DNA ends. Ciprofloxacin, a more potent quinolone, released DNA breaks and killed cells even in the presence of chloramphenicol. It is proposed that this second, chloramphenicol-insensitive mode for release of DNA breaks and cell killing arises from dissociation of gyrase subunits. Ciprofloxacin also killed a gyrase (gyrA) mutant resistant to the prototype of quinolone, nalidixic acid, and created complexes on DNA detected by DNA fragmentation. This lethal effect of ciprofloxacin was eliminated by additional mutations mapping in parC, one of the two genes encoding topoisomerase IV. Thus, the fluoroquinolone compounds have two intracellular targets. In the absence of the gyrA mutation, the parC (CipR) allele did not by itself confer resistance to ciprofloxacin, indicating that gyrase is the major quinolone target in E. coli. These findings provide a molecular explanation for quinolone action in bacteria and a new way to study topoisomerase IV-chromosome interactions.