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Background and purpose: Presence of extended spectrum β -lactamase (ESBL) genes plays an important role in spreading β -lactam antibiotic resistance in the producing strains of these enzymes. The resistance of gram-negative bacteria, such as Escherichia coli, to different antimicrobial agents, has increasingly been reported. This study was conducted to determine the prevalence of ESBL in Escherichia coli isolates Material and Methods: This analytical cross-sectional study was carried out between March 2013 and February 2014 on 131 bacterial of Escherichia coli which were isolated from clinical patient specimens in general hospitals in Urmia (northwest Iran). The frequency of ESBL producing strains was determined via the combined disk method. The presence of β- lactamase gene of CTX-M-15 in ESBL was assessed by PCR method. Results: clinical presentations of infection were UTI in 108 cases (82.44%), septicemia in 15 cases (11.45%), wound infection in 7 cases (5.34%), and meningitis in 1 case (0.76%). Frequency of ESBL positive in Escherichia coli isolates was 55 cases (41.98%) of which 23.63% were positive for CTX-M-15 β -lactamases resistance gene. Conclusion: The rate of ESBLs producing strains is highly increasing, therefore, using an appropriate treatment protocol based on the antibiogram pattern of the strains is highly recommended