ThAims: This study evaluated the relationship between Campylobacter susceptibility and enteric fluoroquinolone concentrations in chickens treated with different doses of enrofloxacin. Methods and Results: All chickens were challenged with seven fluoroquinolone sensitive Campylobacter jejuni ( 6.6 x 10(6) CFU per bird) at 2 weeks posthatch. At 26 days of age chickens were treated with 0 ( n = 29 birds), 25 mg ml(-1) enrofloxacin ( Baytril (R), Bayer Corp., Shawnee Mission, KS, USA) for 3 days ( n 45 birds) or 50 mg ml(-1) enrofloxacin for 7 days ( n 65 birds) in the drinking water. The crop, upper ileum, lower ileum, ceca and colon contents were collected from both enrofloxacin treatment groups ( n 5 birds per day per treatment group) and nonmedicated controls. The minimum inhibitory concentration ( MIC) of ciprofloxacin for Campylobacter increased for isolates from both treatment groups within the first day of dosing and the daily average ranged from 1.4 to 6.5 mu g ml(-1) throughout the study. Although enteric fluoroquinolone concentrations were higher ( P < 0.05) in birds dosed with 50 mg ml(-1) vs 25 mg ml) 1 enrofloxacin, there were no differences between the isolates collected from these groups for MIC values. Conclusion: These data indicate, for the doses used, differences in gut fluoroquinolone concentrations do not produce isolates of Campylobacter with differing susceptibility to ciprofloxacin. Significance and Impact of the Study: Using the manufacturers lowest, shortest duration dose vs the highest, longest duration dose of enrofloxacin did not change Campylobacter susceptibility to ciprofloxacin. However, ciprofloxacin MIC values for Campylobacter determined in this study were lower than previously reported.