Capsulate Haemophilus influenzae is a major cause of septicemia and meningitis in children. Virtually all invasive strains have a type b polysaccharide capsule and belong to division I of the two phylogenetic divisions into which the H. influenzae population segregates. In this study 18 isolates, collected from all over the world and representative of the whole population of division I type b strains, have been shown to be the progeny of a common ancestor in which a founder mutation occurred, the deletion of one of two copies of the gene bexA. BexA is essential for exporting capsular polysaccharide to the bacterial surface, and a single copy of its gene lies at the center of an otherwise duplicated capsulation locus. Deletion of the other copy has had the paradoxical effect of enhancing pathogenicity, through increasing the potential for amplification of capsule biosynthetic genes and capsule production.