Objectives: Avian colibacillosis is responsible for large economic losses in poultry industry. Antimicrobial therapy is an important tool in reducing both the incidence and mortality associated with avian colibacillosis. Antimicrobial-resistant Escherichia coli can be transferred from animals to humans through consumption of contaminated food and it causes a public health risk. The present study was conducted to estimate the antimicrobial resistance of Escherichia coli isolated from broiler chicken in North and Northwest of Iran. Materials & Methods: A total of 109 isolates of E. colifrom North of Iran and58 isolates of E. Colifrom Northwest of Iran were collected since 2012 to 2013, and were analyzed for resistance to 8 antimicrobial agents. Statistical analysis was carried out using SPSS 16.0 where applicable. Results &Conclusion:Antibiograms revealed that the highest level of resistance was observed against Sulfamethoxazole+trimethoprim (Sultrim®)(74.3%) and Enrofloxacin (59.6%) in north;Sulfamethoxazole+trimethoprim (Sultrim®) (24.2%) and Doxycycline(19%) in Northwest of Iran.The most sensitive antibiotics in North were Colistin and Lincomycin+Spectinomycin (Lincospectine®)(10.2% and 22.1% respectively); and in Northwest Fosfomycin (Fosbac®) and Gentamycin(1.7% and 3.4% respectively). Multi drug resistance was also detected in high percentage of E.coli isolates. There were some differences in susceptibility patern of E.coli isolates between seasons of year. This study showed that very high level of resistance to some common antibiotics in poultry farms causes treatment failure, because they do not have remedial effects.

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