A highly discriminative and objective genetic characterization of N. gonorrhoeae, which increases our knowledge of strain populations in different geographic areas, is crucial for the development of improved control measures. In the present study, conventional phenotypic characterization and genetic characterization by means of pulsed-field gel electrophoresis (PFGE), sequencing of the entire porB gene, N. gonorrhoeae multiantigen sequence typing (NG-MAST), and pyrosequencing of a quinolone resistance determining region (QRDR) of the gyrA gene of Swedish ciprofloxacinresistant N. gonorrhoeae serovar IB-10 isolates (n=45) were performed. The genetic characterization identified one widely spread ciprofloxacin-resistant N. gonorrhoeae ST147 strain. In addition, isolates with slightly different genetic characteristics, which presumably reflect the ongoing evolution only, were also identified. All the isolates contained single nucleotide polymorphisms in QRDR of the gyrA gene that are highly correlated with ciprofloxacin resistance. Consequently, comprehensive characterization identified the first confirmed large domestic transmission, mainly among young heterosexuals, of one ciprofloxacin-resistant N. gonorrhoeae strain in Swedish society during [2002][2003]. In conclusion, a precise, i.e. genetic, characterization for identification of individual strains is a very valuable support to the crucial active surveillance of the epidemiological characteristics and the antibiotic susceptibility of N. gonorrhoeae in the effective treatment of gonorrhoea. KeywordsNeisseria gonorrhoeae; ciprofloxacin resistance; molecular epidemiology; porB gene; NG-MAST Gonorrhoea remains a major sexually transmitted infection (STI) in many countries (1,2). The mainstay in its prevention is based on the availability of effective diagnostics and antibiotic treatment, as well as regional, national, and international surveillance of the epidemiological characteristics and the antibiotic susceptibility of the etiological agent, i.e. Neisseria gonorrhoeae. Nowadays, a high level of decreased susceptibility or resistance to most traditional antibiotics used for treatment of gonorrhoea has been identified in most countries [2][3][4][5]). In addition, the characteristics of circulating N. gonorrhoeae strains fluctuate over time and a highly discriminative and objective characterization, i.e. genetic typing, of the bacteria, is crucial. This genetic typing may identify the transmission of more virulent and/or antibiotic-resistant strains, information that is important for the development of improved control measures. Conventional phenotypic characterization of N. gonorrhoeae strains has several limitations (6-10) and many genetic methods have been developed to overcome these shortcomings (11).Since 1997 the incidence of gonorrhoea in Sweden has increased almost annually (Gonorrhoea/ year and month statistics per county http://gis.smittskyddsinstitutet.se/mapapp/build/intro_eng.html (3,8)). According to previous studies (8,10,12), the transmi...