To study the role of the group A streptococcal capsule in pharyngeal colonization, we used two acapsular mutants derived from a type 24 strain of group A Streptococcus by transposon mutagenesis. One mutant had a stable acapsular phenotype due to a transposon-associated chromosomal deletion of essential capsule synthetic genes, while the second mutant could revert to the encapsulated phenotype at a low frequency (< 10(-4)) upon spontaneous excision of the transposon from the capsule-synthesis region of the chromosome. Both acapsular mutants were sensitive to phagocytic killing in vitro and had reduced virulence in mice after intraperitoneal challenge. Mice inoculated intranasally with the stable acapsular mutant rapidly cleared the organisms from the pharynx, and no mice died. In contrast, throat cultures of animals challenged with the revertible mutant yielded many encapsulated revertants, and mortality was similar to that of animals challenged with the parent strain. The rapid emergence of a population of encapsulated revertants in the pharynx implies that the capsule conferred a powerful selective advantage in this environmental niche. Together with the complete avirulence of the stable acapsular mutant, these observations indicate that the hyaluronic acid capsule plays a critical role in colonization and infection of the pharynx by group A streptococci.