One-hundred sixty-eight group B streptococcal (GBS) isolates from a Brazilian hospital were phenotypically and genotypically characterized. Isolates were recovered from human sources from April 2006 to May 2008 and classified as either invasive, noninvasive, or colonizing isolates. Classical methods for serotyping and antibiotic resistance profiling were employed. Clonal groups were also defined by pulsed-field gel electrophoresis (PFGE). Results showed that susceptibility to beta-lactam antimicrobials was predominant among the isolates. Only 4.7% were resistant to erythromycin and clindamycin. The erm(B) gene was widely detected in our GBS isolates, according to our phenotypic results (constitutive macrolide-lincosamide-streptogramin B [cMLS B ] resistance phenotype), and the erm(A) gene was also detected in some isolates. MLS B resistance was restricted to strains isolated from patients with noninvasive infections and carriers. Serotype Ia was predominant (38.1%), serotype IV isolates were found at a high frequency (13.1%), and few isolates of serotype III were identified (3%). Pulsed-field gel electrophoresis results revealed a variety of types, reflecting the substantial genetic diversity among GBS strains, although a great number of isolates could be clustered into two major groups with a high degree of genetic relatedness. Three main PFGE clonal groups were found, and isolates sharing the same PFGE type were grouped into different serotypes. Furthermore, in a few cases, isolates from the same patients and possessing the same PFGE type were of different serotypes. These findings could be related to the occurrence of capsular switching by horizontal transfer of capsular genes.In the last 40 years, Streptococcus agalactiae (group B streptococcus [GBS]) has been described as an important pathogen in neonates and pregnant women. Vaginal colonization with GBS during pregnancy is significantly associated with infections in newborns and requires investigation (40,46,48). Despite the reduction in the incidence of early-onset neonatal disease by using antimicrobial intrapartum prophylaxis, mortality and permanent disability rates caused by GBS continue to be significant. However, GBS has also emerged as an important pathogen in other patient groups, such as children, young adults with underlying medical conditions, and elderly individuals (40). Penicillin is the drug of choice for prevention and treatment of GBS infections, which remain universally susceptible. Erythromycin and clindamycin are recommended when risks of anaphylaxis or therapeutic failure are present. However, resistance to erythromycin and clindamycin has increased in many countries in North America (2,6,16,40), Europe (21,22,42,45), and Asia (24) but not in Brazil (18,50) and other Latin American countries (23,35,36). Such a resistance profile is mainly due to two mechanisms: a methylasemediated target site modification and an active efflux pump (53). In GBS, coresistance to macrolide, lincosamide, and streptogramin B (MLS B ) is due to methylases e...