We report the description of a new plasmid-encoded AmpC-type beta-lactamase gene (bla(CMY-11)) from Escherichia coli K983802.1 that was isolated from a patient in South Korea suffering from a urinary tract infection. Antibiotic susceptibility testing, plasmid analysis, pI determination, transconjugation and Southern blot analysis were carried out to investigate the resistance mechanism to cefoxitin. PCR, sequencing and sequence analysis were used to identify and analyse the beta-lactamase gene (bla(CMY-11)) responsible for the cefoxitin resistance. CMY-11 and bla(CMY-11) are compared with other class C beta-lactamases and their genes to determine phylogenetic relationships. The cefoxitin-resistance phenotype of E. coli K983802.1 reflects the presence of a large plasmid [pYMG-2 (130 kb)]. A beta-lactamase with a pI value of 8.0 from a transconjugant of E. coli K983802.1 was identified by isoelectric focusing. A 1478 bp DNA fragment from pYMG-2 containing bla(CMY-11) was sequenced and an open reading frame coding for a 382 amino acid peptide (CMY-11) was found. Phylogenetic analysis clearly shows that bla(CMY-11) belongs to the group of ampC-related bla genes. It is likely that bla(CMY-11) evolved from bla(CMY-1) via bla(CMY-10).