Ciprofloxacin and other newer quinolone antimicrobial agents exhibit increased potency and decreased frequency of spontaneous bacterial resistance in comparison with older analogues such as nalidixic acid. New and published observations on the mechanisms of action of and resistance to ciprofloxacin in Escherichia coli are presented and discussed. Genetic and biochemical studies have identified the A subunit of the essential bacterial enzyme DNA gyrase as a target of ciprofloxacin and other quinolones. For a series of quinolones, inhibition of purified DNA gyrase correlated with antibacterial activity. The bactericidal activity of ciprofloxacin and ofloxacin is, in contrast to that of certain other quinolones, somewhat less affected by rifampin and cell starvation, suggesting the existence of a site of drug action in addition to DNA gyrase. The frequency of selection of spontaneous single-step resistance mutants of E. coli was more than 100-fold lower with ciprofloxacin than with nalidixic acid. Strains highly resistant to ciprofloxacin could, nevertheless, be selected by serial passage on drug-containing agar. Two mutations contributing to this high level of resistance were analyzed. One, designated cfxA, conferred a 16-fold increase in drug resistance and mapped in a location consistent with a gyrA mutation; similar increases in resistance to ciprofloxacin were seen with gyrA mutations selected for resistance to other quinolones. The other mutation, cfxB, conferred pleiotropic resistance to ciprofloxacin, tetracycline, and chloramphenicol and appeared to be an allele of the multiple antibiotic resistance gene marA. The mutation cfxB was associated with a decreased amount of porin outer membrane protein OmpF, suggesting that drug permeation may occur in part through this channel. In summary, the A subunit of DNA gyrase is a target of ciprofloxacin and other quinolones. Ciprofloxacin resistance appears to occur both by mutation in this target and by alteration of drug permeation through the outer membrane of the cell.