Pseudomonas aeruginosa is a well-known cause of severe and potentially life-threatening infections including bacteremia, skin and wound infections, pulmonary disease, especially among indivuals with cycstic fibrosis, nosocomial urinary tract infections, endocarditis and meningitis. The mechanism of resistance to broad-spectrum beta-lactams in P.aeruginosa are overexpression of cephalosporinases and/or class A, B and D beta-lactamases. Recently PER-1 type beta-lactamase has been reported from Turkey, France, Italy, Romania, Hungary, Belgium, Russia, South Korea and India. OXA beta-lactamases have increasingly been reported in clinical strains of P.aeruginosa from various geographical origins. This study was aimed to investigate the antibiotic susceptibility of various P.aeruginosa clinical strains and to define the beta-lactamase enzymes leading to resistance. In this study, a total of 100 P.aeruginosa strains isolated from various clinical specimens (37 urine, 21 blood, 10 sputum, 5 bronchoalveolar lavage, 5 abscess, 5 wound swabs, 4 endotracheal aspirate, 3 throat swabs, 2 catheter tips, one of each pleural and peritoneal fluid) were included. According to Clinical and Laboratory Standards Institute (CLSI) recommendations, susceptibilities of isolates to various antibiotics were investigated by disk diffusion and agar dilution method, and beta-lactamase enzymes were detected by isoelectric focusing (IEF) method. PSE, PER-1, OXA-10-like beta-lactamase genes and MEX-R genes of isolates were investigated by polymerase chain reaction (PCR). According to MIC90 values, the most effective antibiotics were found to be imipenem (8 mu g/ml). The MIC90 values of amikacin, ciprofloxacin, cefepime, cefpirome, piperacillin + tazobactam, piperacillin, ceftazidime, ticarcilin, aztreonam and ticarcilin + clavulanic acid were 32, 64, 64, 64, 128/4, 512, 512, 512, 512 and 512/2 mu g/ml, respectively. Seven of the isolates were found to be ESBL positive by double-disk synergy method. It was detected that 10% of the isolates were imipenem-susceptible and 9% were intermediate susceptible. Phenotypical investigation of metallo-beta-lactamase enzyme in these strains by MBL E-test method did not reveal a positive result. PER-1 and OXA-10 like beta-lactamases were detected each in 11% of the isolates, and co-presence of PER-like and OXA-10 like enzymes were shown in 4% of the isolates. PSE gene was not found in any of the strains. The MEX-R gene was identified in 52% of the isolates. Antibiotic resistance mechanisms in P.aeruginosa strains seems to be complex. Determination of the resistance mechanisms and antibiotic susceptibility rates in P.aeruginosa will guide the proper antimicrobial therapy, reducing the emergence of resistant strains.