This study aimed at comparing four different methods designed for detection of extended spectrum beta-lactamase (ESBL) production. Eighty-nine Klebsiella pneumoniae, seven Klebsiella oxytoca and 63 Echerichia coli strains which were isolated from the clinical specimens of ambulatory and hospitalized patients were studied to identify ESBL production by a) evaluating the resistance patterns of ceftazidime, aztreonam, cefotaxime and ceftriaxone in disk diffusion test (RP), b) comparing the disk diffusion zones of these antibiotics (ZC), c) double disk synergy tests (with 25 mm and 30 mm disk distances) (DDS25 and DDS30) and d) E-test (for Klebsiella spp. only). The highest rate for ESBL production was observed in K. pneumoniae strains (79%) and by ZC method, with no significant difference between the ambulatory and hospitalized patients (p=0.05). ESBL production was less frequent in E. coli strains by this method (11%) (p<0.001). Spearman correlation test revealed very good correlation between RP and DDS25 (r=0.77), moderate correlation between ZC and E-test (r=0.40) and good correlation between the other methods (r=0.64-0.74).