The frequency of the extended spectrum beta lactamases (ESBL) was investigated in Klebsiella pneumoniae, Klebsiella oxytoca and Escherichia coli strains isolated from clinical specimens in the Emergency Laboratory of the Department of Microbiology and Clinical Microbiology, Istanbul Faculty of Medicine, Istanbul University. 48% of the K.pneumoniae, 40% of the K.oxytoca and 14% of the E.coli strains were found as ESBL producers. The total rate for ESBL positive isolates was 28%.