Aims: We report the description of a novel plasmid-encoded AmpC beta-lactamase gene (bla(CMY-10)) from Enterobacter aerogenes K9911729 that was isolated from a patient suffering from pneumonia in South Korea. Methods and Results: Using antibiotic susceptibility testing, plasmid analysis, transconjugation and Southern blot analysis, the cefoxitin resistance phenotype reflects the presence of a large plasmid [pYMG-1 (130 kb)] in Ent. aerogenes K9911729. One beta-lactamase with the pI of 8.0 from transconjugant of Ent. aerogenes K9911729 was identified by isoelectric focusing on a gel. A 1475 bp DNA fragment containing the bla(CMY-10) gene, identified on pYMG-1 of Ent. aerogenes K9911729, was sequenced and an open reading frame coding for 382 amino acid, CMY-10, was found. The 37 class C beta-lactamases were subclassified into 1a to 1j and CMY-10 into 1a by phylogenetic analysis. A sequence identical to the common regions in In6, In7 and a novel integron from pSAL-1 was found upstream from bla(CMY-10) gene at nucleotide 1 - 71. Conclusions: These results clearly show that bla(CMY-10) gene belongs to the group of ampC-related bla genes. Homology analysis among AmpC enzymes or ampC genes implied that integration of the chromosomal ampC gene into a large resident plasmid, followed by transconjugation, was involved in the evolution of bla(CMY-10) gene. Significance and Impact of the Study: The first identification of the bla(CMY-10) gene is of concern as chromosomal beta-lactamases may cause serious therapeutic problems if their genes are translocated onto plasmids.