Abstract INTRODUCTION: Pseudomonas aeruginosa is one of the most common nosocomial pathogens. The emergence of extended spectrum β-lactamases (ESBLs) has been increasingly reported as a major clinical concern worldwide. The main aim of the present study was to determine the distribution of bla OXA, bla PER-1, bla VEB-1, and bla GES-1 genes among ESBL-producing P. aeruginosa isolated from two distinct provinces in Iran. METHODS: In this study, a total of 75 (27.5%) ESBL-producing isolates were identified from 273 P. aeruginosa isolates collected from patients in Qazvin and Tehran. Phenotypic detection of ESBLs and antimicrobial susceptibility testing were performed according to the Clinical and Laboratory Standards Institute guidelines. PCR and sequencing were employed to detect bla OXA-1, bla OXA, bla GES-1, bla PER-1, and bla VEB-1 genes. Isolate genetic relationships were evaluated by repetitive extragenic palindromic sequence-based PCR (REP-PCR). RESULTS: In total, 59 (78.7%) of the ESBL-producing isolates showed multidrug resistance. The highest rates of susceptibility were observed against colistin (75 isolates, 100%) and polymyxin B (75, 100%) followed by amikacin (44, 58.7%), and piperacillin-tazobactam (40, 53.3%). The bla OXA-1 (37.3%) gene was the most common of the genes investigated, followed by bla OXA-4 (32%), bla GES-1 (16%), and bla VEB-1 (13.3%). REP-PCR identified three different genotypes: types A (89.3%), B (6.7%), and C (4%). CONCLUSIONS: We found a significant presence of bla OXA-1, bla OXA-4, bla GES-1, and bla VEB-1 genes among P. aeruginosa isolates, highlighting the need for suitable infection control strategies to effectively treat patients and prevent the further distribution of these resistant organisms.