Among 2133 isolates of Escherichia coli obtained during 1996 from 10 Greek hospitals, 63 (3%) were resistant to cefoxitin. Typing by ERIC2-PCR indicated that the cefoxitin-resistant (FOXr) isolates were distinct. beta-Lactamase studies and hybridization experiments showed that most strains produced beta-lactamases related to the AmpC chromosomal cephalosporinase of Citrobacter freundii. The enzymes were encoded by similar non-self-transmissible plasmids. The bla genes encoding two beta-lactamases (LAT-3 and LAT-4) with isoelectric points 8.9 and 9.4, respectively, were cloned and sequenced. The deduced amino acid sequences displayed a high degree of homology (>95%) with the AmpC beta-lactamase of C. freundii. The patterns of resistance to beta-lactams of the FOXr E. coli depended on the quantity of class C enzymes and the simultaneous expression of other beta-lactamases. In a few isolates a 36 kDa outer-membrane protein, presumably a porin, was not expressed at detectable quantities. These isolates were resistant to cefoxitin, and their susceptibility to the other beta-lactams tested was not significantly decreased.