The prevalence of extended-spectrum β-lactamases (ESBL) was determined among isolates ofEscherichia coli (n=63) isolated from hospitalized (43) and healthy (20) children. Ten isolates (21 %) were ESBL-positive for two screening tests, the double disk-synergy test and theOxoid Combination Disk method. One ESBL-positive isolate came from a healthy child. The transfer frequency of oxyimino-β-lactam resistance from ESBL-producing isolates toE. coli K12 C600 recipient strain ranged from 10−8 to 10−5 per donor cell. Donor strains and transconjugants displayed susceptibility patterns typical of ESBL producers. They were resistant to oxyimino-β-lactams but susceptible to clavulanic acid and carbapenems. Seven out of the 10 ESBL-positive isolates were found to produce MR/MS fimbria, which may play an important role in the colonization of the human intestinal mucosa.