Proteus mirabilis isolates obtained in 1999 to 2008 from three European countries were analyzed; all carried chromosomal AmpC-type cephalosporinase bla(CMY) genes from a Citrobacter freundii origin (bla(CMY-2)-like genes). Isolates from Poland harbored several bla(CMY) genes (bla(CMY-4), bla(CMY-12), bla(CMY-14), bla(CMY-15), and bla(CMY-38) and the new gene bla(CMY-45)), while isolates from Italy and Greece harbored bla(CMY-16) only. Earlier isolates with bla(CMY-4) or bla(CMY-12), recovered in France from Greek and Algerian patients, were also studied. All isolates showed striking similarities. Their bla(CMY) genes resided within ISEcp1 transposition modules, named Tn6093, characterized by a 110-bp distance between ISEcp1 and bla(CMY), and identical fragments of both C. freundii DNA and a ColE1-type plasmid backbone. Moreover, these modules were inserted into the same chromosomal site, within the pepQ gene. Since ColE1 plasmids carrying ISEcp1 with similar C. freundii DNA fragments (Tn6114) had been identified earlier, it is likely that a similar molecule had mediated at some stage this DNA transfer between C. freundii and P. mirabilis. In addition, isolates with bla(CMY-12), bla(CMY-15), and bla(CMY-38) genes harbored a second bla(CMY) copy within a shorter ISEcp1 module (Tn6113), always inserted downstream of the ppiD gene. Sequence analysis of all mobile bla(CMY-2)-like genes indicated that those integrated in the P. mirabilis chromosome form a distinct cluster that may have evolved by the stepwise accumulation of mutations. All of these observations, coupled to strain typing data, suggest that the bla(CMY) genes studied here may have originated from a single ISEcp1-mediated mobilization-transfer-integration process, followed by the spread and evolution of a P. mirabilis clone over time and a large geographic area.