BACKGROUND: Enterobacter species are frequent nosocomial pathogens and the proportion of beta-lactam resistant strains are on the increase. Extended spectrum beta-lactamases(ESBLs) were mainly investigated in Klebsiella pneumoniae and Escherichia coli in Korea. Recently, we experienced 4 strains of multidrug(including cephamycin)-resistant Enterobacter cloacae and characterized the ESBL types. METHODS: Multidrug-resistant E. cloacae strains were tested for ESBL production by double-disk synergy test and conjugation. The presence of TEM, SHV or IMP gene was determined by polymerase chain reaction. RESULTS: Of the four strains that revealed positive reaction in double-disk synergy test, ceftazidime- resistance was transferred in two and cefoxitin-resistance was transferred in four strains by conjugation. In the polymerase chain reaction, three out of four strains had both TEM and SHV genes and one strain had only TEM gene. Two ceftazidime transconjugants had both TEM and SHV genes. CONCLUSION: We should be aware of ESBL producing Enterobacteriaceae and the possible institutional spread of resistance genes.