Streptococcus mutans is known to be one of the most cariogenic oral microorganisms. Recently, organisms classified as S. mutans have been shown to be genotypically heterogeneous and to form at least six separate species. In this study, differences in acid production between the mutans streptococci and Streptococcus sanguis were investigated at pH values from 5.0 to 7.0. While all of the species investigated were able to produce acids, Streptococcus sobrinus was found to be significantly more acidogenic than the others. S. sobrinus was capable of sustained acid production at pH levels below 6.0, while acid production by other species ceased or was strongly inhibited. These results indicate that S. sobrinus deserves more attention as a potentially cariogenic microorganism.