Nationwide, CTX-M-producing clinical Escherichia coli isolates from the Norwegian ESBL study in 2003 (n=45) were characterized on strain and plasmid levels. Bla(CTX-M) allele typing, characterization of the genetic environment, phylogenetic groups, pulsed field gel electrophoresis (PFGE), serotyping and multilocus sequence typing were performed. Plasmid analysis included S1-nuclease-PFGE, polymerase chain reaction-based replicon typing, plasmid transfer and multidrug resistance profiling. Bla(CTX-M-15) (n=23; 51%) and bla(CTX-M-14) (n=11; 24%) were the major alleles of which 18 (78%) and 6 (55%), respectively, were linked to ISEcp1. Thirty-two isolates were of phylogenetic groups B2 and D. Isolates were of 29 different XbaI-PFGE-types including six regional clusters. Twenty-three different O:H serotypes were found, dominated by O25:H4 (n=9, 20%) and O102:H6 (n=9, 20%). Nineteen different STs were identified, where ST131 (n=9, 20%) and ST964 (n=7, 16%) were dominant. Bla(CTX-M) was found on > or =100 kb plasmids (39/45) of 10 different replicons dominated by IncFII (n=39, 87%), FIB (n=20, 44%) and FIA (n=19, 42%). Thirty-nine isolates (87%) displayed co-resistance to other classes of antibiotics. A transferable CTX-M phenotype was observed in 9/14 isolates. This study reveals that the majority of CTX-M-15-expressing strains in Norway are part of the global spread of multidrug-resistant ST131 and ST-complex 405, associated with ISEcp1 on transferrable IncFII plasmids.