Capsule switching in Neisseria meningitidis is thought to occur by horizontal DNA exchange between meningococcal strains. Antigenic variants may be generated by allelic replacement of the siaD gene; the variants may then be selected by specific immunity against the capsular antigen. There were several vaccination campaigns against serogroup C in France in 2002, following an increase in the prevalence of invasive isolates of serogroup C of the phenotype C:2a:P1.5 and C:2a:P1.5,2 belonging to the ET-37/ST-11 clonal complex. We evaluated the emergence of capsule variants by the detection of B:2a:P1.5 and B:2a:P1.5,2 meningococcal isolates of the ET-37/ST-11 clonal complex. These isolates were significantly more frequent after the year 2002. Pulsed field gel electrophoresis profiles of the serogroup B (ET-37/ST-11) isolates differed from that of serogroup C (ET-37/ST-11) isolates by the bands that harbor the siaD genes responsible for the serogroup specificity. However, serogroup B and C, ET37/ST-11 isolates both express similar virulence as assessed from colonization and invasiveness in a mouse model. Our results indicate that capsule switching events within the same clonal complex can arise frequently with no alteration in virulence. This justifies an enhanced system of surveillance by molecular typing of such isolates, particularly after serogroup-specific vaccination.