Aims To determine whether therapeutic treatment of pigs with chlortetracycline affects the susceptibility of their Campylobacter isolates for tetracycline, ciprofloxacin and erythromycin. Methods and Results Minimum inhibitory concentrations (MICs) and presence of a tetracycline resistance gene tet(O) were studied in Campylobacter collected before, during and after chlortetracycline treatment. Tetracycline MICs and the presence of tet(O) for additional Campylobacter coli isolates collected previously from seven farrowing farms were also determined. Isolates with ciprofloxacin MICs above the epidemiological cut-off value (ECOFF) were subtyped by flaA restriction fragment length polymorphism (RFLP). Tetracycline MICs of 221 Camp.coli isolates remained under the ECOFF at all sampling stages as well as the MICs for 63 isolates from the other farms. The ciprofloxacin MIC was above the ECOFF for 22% of the isolates, and one Camp.coli isolate had an erythromycin MIC above the ECOFF. None of the studied 300 Campylobacter isolates from nine herds carried tet(O). flaA-RFLP typing revealed the heterogeneity of Camp.coli isolates with high ciprofloxacin MICs. Conclusion Use of chlortetracycline did not increase the MIC values for the antimicrobials studied. Significance and Impact of the Study This study demonstrated that susceptibility of Camp.coli isolates is not affected by chlortetracycline therapy if tet(O) is not present in Camp.coli population.