Objective: The aim of this study was to characterize the CTX-M genes in clinical isolates of Klebsiella pneumoniae, particularly, the plasmid types that carry them. Material and Methods: Antimicrobial susceptibilities were determined by the agar dilution and E-test. Beta-lactamase production as analyzed by phenotypic tests (E-test MBL and extended spectrum beta-lactamase (ESBL), isoelectric focusing, and bioassay] and molecular methods [polymerase chain reaction (PCR) detection of ESBL-encoding genes and IS elements, DNA sequencing and analysis of blaCTX-M and ISEcP1 PCR amplicons, typing by randomly amplified polymorphic DNA (RAPD) analysis and plasmid isolation, transformation, rep-typing and IncN plasmid confirmation]. Results: Thirty four (14.8%) out of 230 clinical isolates of K. pneumoniae were found as ESBL producers. The isolates produced one to five different beta-lactamases, according to the isoelectric focusing results. The prevalence of the CTX-M-type ESBLs was found as 35% and sequencing proved all as CTX-M-15. RAPD analysis showed no clonal relation between the strains. Previous studies have shown that the blaCTX-M-15 gene was carried on FII plasmids. In 10 strains in this study, the CTX-M-15 gene was on 95 kb-larger plasmids typing to IncN. In two isolates the blaCTX-M-15 was carried on an approximately 60 kb plasmid and possessed an Inc/rep type of FII. Conclusion: This is the first report of IncN carrying blaCTX-M-15 and confirms the rapid emergence of CTX-M-15 enzymes among K. pneumoniae in Istanbul. Through this study, it was aimed to underline the risk of spread of IncN type plasmids, among gram-negative bacteria in Turkey, as shown previously in Greece.