ABSTRACTWe analyzed 212 group B streptococci (GBS) from newborns with invasive infections in the area of Barcelona, Spain, between 1992 and 2009, with the aim of documenting changes in the prevalences of serotypes, antimicrobial resistance, and genetic lineages and evaluating their associations with either early-onset disease (EOD) or late-onset disease (LOD). Serotypes III (n= 118) and Ia (n= 47) together accounted for nearly 78% of the isolates. All isolates carried an alpha or alpha-like protein gene, and specific associations between genes and serotypes, such as serotype Ib andbca, serotype II andbca, serotype III andrib, and serotype V andalp3, reflected the presence of particular genetic lineages. Macrolide resistance (14.2%) was significantly associated with serotype V. Pulsed-field gel electrophoresis (PFGE) clustering was an excellent predictor of serotype and antibiotic resistance. The combination of PFGE and multilocus sequence typing revealed a large number of genetically distinct lineages. Still, specific lineages were dominant in our collection, particularly the serotype III/ST17/riblineage, which had enhanced potential to cause LOD. Serotype Ia was concentrated in a single PFGE cluster composed of two genetic lineages: ST23/epsand ST24/bca. The ST24/bcasublineage of serotype Ia, which is found infrequently elsewhere, may be emerging as an important cause of neonatal invasive infections in the Mediterranean region. In spite of the introduction of prophylaxis, resulting in a pronounced decline in the frequency of EOD, the study revealed a remarkably stable clonal structure of GBS causing neonatal infections in Barcelona over a period of 18 years.