Cefoxitin-resistant Escherichia coli and Klebsiella pneumoniae are relatively prevalent in Korea, suggesting dissemination of plasmid-mediated AmpC beta-lactamases. In this study, 238 isolates of cefoxitin-resistant E. coli and K. pneumoniae (not including subspecies ozaenae and rhinoscleromatis) were collected in 2003 from 16 Korean hospitals. The prevalence of plasmid-mediated AmpC beta-lactamases was determined by PCR. The AmpC gene alleles detected in E. coli and K. pneumoniae were bla(DHA-1), 10 (8.6%) and 93 (76.2%); bla(CMY-1)-like, 14 (12.1%) and 2 (1.6%); and bla(CMY-2)-like, 38 (32.7%) and 1 (0.8%) isolates, respectively. The genes identified were bla(DHA-1), bla(CMY10)-like, and bla(CMY-2)-like, and a new variant, bla(CMY-18). Plasmidmediated AmpC gene allele-positive isolates were present both in large city and in small province hospitals, as well as in isolates from outpatients. The proportions of plasmid-mediated AmpC gene-positive isolates were similar in both expanded spectrum beta-lactamase (ESBL)-producing and -nonproducing isolates. In conclusion, DHA-1, CMY-2-like, and CMY-10-like plasmid-mediated AmpC beta-lactamase-producing K. pneumoniae and E. coli isolates are widely disseminated in both large city and small province hospitals. Absence of bla(CMY-1) and detection of a novel variant of bla(CMY-2), bla(CMY-18), indicate continued evolution of the prototype genes. Similar proportions of plasmid-mediated AmpC gene-positive isolates in both ESBL-producing and -nonproducing isolates suggest unhindered future spread of these resistances.