Escherichia coli isolates (n=438) from six different broiler chicken flocks (all in, all out) with known consumption of antimicrobials were investigated for their antimicrobial resistance and the prevalence of extended-spectrum β-lactamase (ESBL) phenotypes. E. coli were isolated from chicken at the third and fifth week of age and tested for antimicrobial resistance during the course of fattening. Resistance to sulfamethoxazole+trimethoprim, which was used in four flocks within the first days of life, decreased significantly in all six flocks between the third and fifth week of broiler chicken's life (mean 65.9% vs. 54.3%). By contrast, resistance to spectinomycin increased significantly in all six flocks within the same period (mean 36.1% vs. 57.0%); doxycycline resistance increased significantly in five of six flocks (mean 19.2% vs. 41.7%), although both substances were not used for treatment. Of the sulfonamide resistance genes sul1, sul2, and sul3, sul2 was most frequently found (up to 60%). The prevalence of sul2 increased significantly between weeks 3 and 5, if the chicken were treated with sulfamethoxazole + trimethoprim in the first days of life. If sulfamethoxazole + trimethoprim was not used, then the prevalence of sul2 decreased significantly in the same period. The prevalence of sul1+qacEΔ1 (classical class 1 integrons) was significantly higher in E. coli from sulfamethoxazole + trimethoprim-treated flocks (9.63%), compared to untreated flocks (2.92%). The detection of phenotypes that potentially indicate plasmid-borne AmpC-β-lactamases was inversely associated with sulfamethoxazole + trimethoprim treatment. ESBL phenotypes were found without selective enrichment in four of six flocks. Of all isolated E. coli, 1.8% (n=8) had an ESBL phenotype. ESBL strains differed in their accompanying resistances and/or enterobacterial repetitive intergenic consensus sequences. In conclusion, clonal dissemination seems not to be a major cause of ESBL detection on a chicken farm with all-in all-out production mode.