The purpose of the present study was to determine the presence of extended spectrum beta-lactamase (ESBL) in clinical isolates of Gram-negative bacilli and also the in vitro antibiotic susceptibility of the strains found to carry ESBL. Totally 222 clinical isolates were screened with double disk synergy test. ESBL production was positive in Escherichia coli, Klebsiella spp., Proteus spp., Enterobacter spp., Pseudomonas aeruginosa isolates as 19.4%, 15.9%, 13.7%, 26.7%, 16.7%, respectively. Of the ESBL producing isolates 16 (38%) were isolated from outpatients and 26 (62%) from hospitalised patients. The difference between these grups was significant (p=0.012). The susceptibility to imipenem, gentamycin, amikacin, ciprofloxacin and cefepime were 95.3%, 78.5%, 97.6%, 80.9% and 92.8%, respectively, among the ESBL producing isolates. It is suggested that ESBL must be evaluated routinely in Gram-negative bacilli isolated from nosocomial infections. Carbapenem, amikacin and cefepim were found as the most effective antibiotics against ESBL producing isolates.