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ABSTRACTGroup A Streptococcus(GAS) predominantly exists as a colonizer of the human oropharynx that occasionally breaches epithelial barriers to cause invasive diseases. Despite the frequency of GAS carriage, few investigations into the contributory molecular mechanisms exist. To this end, we identified a naturally occurring polymorphism in the gene encoding the streptococcal collagen-like protein A (SclA) in GAS carrier strains. All previously sequenced invasive serotype M3 GAS possess a premature stop codon in the sclAgene truncating the protein. The carrier polymorphism is predicted to restore SclA function and was infrequently identified by targeted DNA sequencing in invasive strains of the same serotype. We demonstrate that a strain with the carrier sclAallele expressed a full-length SclA protein, while the strain with the invasive sclAallele expressed a truncated variant. An isoallelic mutant invasive strain with the carrier sclAallele exhibited decreased virulence in a mouse model of invasive disease and decreased multiplication in human blood. Further, the isoallelic invasive strain with the carrier sclAallele persisted in the mouse nasopharynx and had increased adherence to cultured epithelial cells. Repair of the premature stop codon in the invasive sclAallele restored the ability to bind the extracellular matrix proteins laminin and cellular fibronectin. These data demonstrate that a mutation in GAS carrier strains increases adherence and decreases virulence and suggest selection against increased adherence in GAS invasive isolates.