To determine and type the extended-spectrum beta -lactamases (ESBLs) among the family Enterobacteriaceae in a medical center, a total of 668 clinical isolates were screened. Of the 668 isolates, the 80 strains were presumptively defined as ESBL producers according to the result of disk method using ESBL marker antibiotics (aztreonam, ceftazidime, and cefoxitin). These 80 strains were retested with the double-disk synergy test (DDST), the E-test ESBL strip, a 5-mug ceftazidime disk, and agar dilution MICs of ceftazidime with and without clavulonic acid. Isoelectric focusing was performed to confirm ESBL production and type the beta -lactamases. By evaluation of the results of all tests used for ESBL detection together with isoelectric focusing, 33 (4.9%) of the 668 isolates were described as ESBL producer. The positive results of the agar dilution test, DDST, the E-test strip, and 5-mug ceftazidime disk were 32, 26, 27, and 26 of the 33 strains, respectively. ESBL positivity was 48.8% in Klebsiella species, 15.4% in Citrobacter species, 4.9% in Enterobacter species and 1.1% in Escherichia coli strains. The ESBL enzymes frequently determined were SHV-2/6-like (pI 7.6), SHV-5-like (pI 8.2), SHV-4-like (pI 7.8), and SHV-3-like (pI 7). SHV-derived enzymes were commonly observed in Klebsiella spp whereas TEM-related enzymes were seen in E. coli strains. The results of this study indicated that SHV-2/6-derived (pI 7.6) ESBL expression among the isolates of the family Enterobacteriaceae is an important problem in our medical center.