CTX-M-10 has been widely disseminated among multiple clones of several species of Enterobacteriaceae, harboring seemingly different plasmids, for over a decade in Ramón y Cajal University Hospital, Madrid, Spain. Cloning and sequencing of a 12.2-kb DNA fragment from plasmid pRYCE21 from Klebsiella pneumoniae strain KP4aC revealed a novel phage-related element immediately upstream of bla(CTX-M-10) conserved among different CTX-M-10-producing strains. This is the first report showing an extended-spectrum-beta-lactamase gene linked to a phage-related element.