Periodontal disease is associated with changes in the composition of the oral microflora, where health-associated oral streptococci decrease while Gram-negative anaerobes predominate in disease. A key feature of periodontal disease-associated anaerobes is their ability to produce hydrogen sulfide (H 2 S) abundantly as a by-product of anaerobic metabolism. So far, H 2 S has been reported to be either cytoprotective or cytotoxic by modulating bacterial antioxidant defense systems. Although oral anaerobes produce large amounts of H 2 S, the potential effects of H 2 S on oral streptococci are currently unknown. The aim of this study was to determine the effects of H 2 S on the survival and biofilm formation of oral streptococci. The growth and biofilm formation of Streptococcus mitis and Streptococcus oralis were inhibited by H 2 S. However, H 2 S did not significantly affect the growth of Streptococcus gordonii or Streptococcus sanguinis. The differential susceptibility of oral streptococci to H 2 S was attributed to differences in the intracellular concentrations of reduced glutathione (GSH). In the absence of GSH, H 2 S elicited its toxicity through an iron-dependent mechanism. Collectively, our results showed that H 2 S exerts antimicrobial effects on certain oral streptococci, potentially contributing to the decrease in health-associated plaque microflora.