A total of 136 strains of Escherichia coli, isolated from rectal swabs of neonates at the neonatal intensive care unit of Pusan National University Hospital during the period from February to April of 2001 and March to April of 2002 were serotyped. The presence of eaeA, aggA, bfpA, astA, LT, and ST genes was test by PCR. Four enteroaggregative E. coli (EAggEC) strains were isolated in 2001 and eight in 2002. In 2001, three strains of enterotoxigenic E. coli (ETEC) were isolated and these strains were tested for antimicrobial susceptibility. ETEC isolates were detected by a reverse passive latex agglutination (RPLA) test for the production of heat-labile enterotoxin. The strains were analyzed by using plasmid profiling, random amplified polymorphic DNA (RAPD), and pulsed field gel electrophoresis (PFGE) methods. The O serotypes could be assigned to 29 isolates (21.3%): O166,6; O167, 5; O86a, O6 and O127a, 4 each; O8, 2; and O28ac, O44, O158, O20, 1 each. There were 107 untypable isolates. EAggEC isolates were typed as O86a in 4, O127a in 4, and untypable in 4 strains. Three isolates of ETEC were typed as O6. The PCR detected aggA gene in 12 strains (8.8%), but bfpA, eaeA, EAST-1, and ST genes were not detected. LT gene was detected in 3 strains (2.2%) by PCR and DNA probe hybridization, LT production was confirmed in those strains by a latex bead aggregation method. Out of the 15 EAggEC and ETEC strains, eleven (80.0%) were multi-drug resistant to more than 3 antibiotics. Thirteen groups were classed by antibiogram and most strains were susceptible to gentamicin, kanamycin, and cefoperazone, but resistant to ampicillin (100%), cephalothin (66.7%), cefuroxime (46.6%), and tetracycline (46.7%). All isolates possessed a 60 kbp plasmid and 15 strains possessed smaller plasmids. Twelve EAggEC and 3 ETEC strains were differentiated into 5 and 3 groups by the plasmid profiling. RAPD and PFGE grouped the EAggEC isolates into 7 and 6 groups, respectively. The RAPD and PFGE patterns matched in 11 isolates (91.7%) among the 12 EAggEC strains, but the plasmid profile analysis and antibiogram showed no correlation. Three ETEC strains showed different plasmid profiles, and RAPD and PFGE patterns.