Viridans group streptococci (VS) from the oral cavity entering the bloodstream may initiate infective endocarditis (IE). We aimed to identify genes expressed in response to a pH increase from slightly acidic (pH 6.2) to neutral (pH 7.3) as encountered by VS entering the bloodstream from the oral cavity. Using a recently developed promoter-screening vector, we isolated five promoter fragments from the genomic DNA of Streptococcus gordonii CH1 responding to this stimulus. No common regulatory sequences were identified in these promoter fragments that could account for the coordinate expression of the corresponding genes. One of the isolated fragments contained the promoter region and 5′ end of a gene highly homologous to the methionine sulfoxide reductase gene (msrA) of various bacterial and eukaryotic species. This gene has been found to be activated in S. gordonii strain V288 in a rabbit model of IE (A. O. Kiliç, M. C. Herzberg, M. W. Meyer, X. Zhao, and L. Tao, Plasmid 42:67–72, 1999). We isolated and characterized the msrA gene of S. gordonii CH1 and constructed a chromosomal insertion mutant. This mutant was more sensitive to hydrogen peroxide, suggesting a role for the streptococcal MsrA in protecting against oxidative stress. Moreover, MsrA appeared to be important for the growth of S. gordonii CH1 under aerobic and anaerobic conditions. Both these properties of MsrA may contribute to the ability of S. gordonii to cause IE.