Five commercial broiler flocks were treated with a fluoroquinolone for a clinically relevant infection. Fresh feces from individual chickens and environmental samples were cultured for campylobacters before, during, and weekly posttreatment until slaughter. Both Campylobacter jejuni and C. coli were isolated during all treatment phases. An increased proportion of quinolone-resistant strains was seen during treatment, and these strains persisted posttreatment. One quinolone-resistant isolate of each species, each serotype, and each phage type from each sample at all treatment phases was examined for its phenotype and mechanism of resistance. Two resistant phenotypes were isolated: Nalr Cipr and Nalr Cip s. The majority (269 of 290) of fluoroquinolone-resistant isolates, whether they were C. jejuni or C. coli, had a mutation in gyrA that resulted in the substitution Thr-86→Ile. The other gyrA mutations detected were Thr-86→Ala (n = 17) and Asp-90→Asn (n = 10). The genotypic variation, based on the silent mutations in gyrA identified by the denaturing high-performance liquid chromatography pattern and DNA sequencing, was used to supplement typing data and provided evidence for both the spread of preexisting resistant strains and the selection of spontaneous resistant mutants in treated flocks. Multidrug resistance was significantly (P < 0.01) associated with resistance to ciprofloxacin. Twenty-five percent (73 of 290) of ciprofloxacin-resistant isolates but only 13% (24 of 179) of susceptible isolates were resistant to three or more unrelated antimicrobial agents. In conclusion, quinolone-resistant campylobacters were isolated from commercial chicken flocks in high numbers following therapy with a veterinary fluoroquinolone. Most ciprofloxacin-resistant isolates had the GyrA substitution Thr-86→Ile. Resistant isolates were isolated from the feces of some flocks up to the point of slaughter, which may have consequences for public health.