Background and Objective: Pseudomonas aeruginosa is the most common pathogen causing nosocomial infections. The aim of this study was to determine the antimicrobial susceptibility pattern and prevalence of ESBLs in clinical and environmental isolates of P. aeruginosa by phenotypic and genotypic techniques. Materials and Methods: In this descriptive study, a total of 100 P. aeruginosa strains isolated from different clinical and environmental specimens were used. The antibiotic resistance pattern to eight antimicrobial agents was determined by disk diffusion method. The ESBLs producing strains were confirmed by double-disk-diffusion test, and the blaTEM-1, blaSHV-1, blaSHV-5, blaCTX-M-1, blaCTX-M-2, blaCTX-M-3, blaCTX-M-9, blaOXA-1, blaGES-1, and blaGES-2 genes were detected by PCR. Results: Piperacillin and ciprofloxacin showed the highest (36%) and the lowest (16%) resistance against the isolates, respectively. Thirty percent of the total isolates were resistant to at least three classes of antibiotics. By double-disk-diffusion test, eight strains (8%) were ESBL positive. According to the PCR results, the blaGES-2, blaSHV-1, blaSHV-5, and blaCTX-M-1 genes were detected in 8, 2, 2, and 1 isolates of ESBLs producing strains respectively. Conclusion: The blaGES-2 gene displayed an expanded hydrolysis profile to the antibiotic imipenem. In fact, this enzyme, which plays an important role in resistance to imipenem, was detected in all ESBLs producing P. aeruginosa strains. This is the first report describing blaGES-2 producing P. aeruginosa in Iran.