Abstract: There is a clear association between heavy antimicrobial consumption in poultry industry and the recovery of resistant bacteria. This was a case-control study of 396 E. coli strains isolated from clinically affected broiler chickens and 132 strains from healthy controls to compare the antimicrobial resistance rates. Antimicrobial resistance testing of 525 avian E. coli strains isolated in Kashan-Iran showed very high levels of resistance to 11 antimicrobials tested, especially to trimethoprim-sulfamethoxazole (98.7%) and to ciprofloxacin (69.7%). The prevalence rate of resistant E. coli to ciprofloxacin and erythromycin in the samples isolated from chickens with colibacillosis was significantly higher than healthy controls. In addition, to prevent the emergence of cross-resistance with human enteric pathogens, controlled use of these antimicrobial agents in veterinary practice is recommended.