We determined the 16S rRNA sequences of the type strains of Streptococcus mitis and Streptococcus gordonii and calculated the phylogenetic distances between those organisms and other members of the genus Streptococcus. The viridans group streptococci were separated into five phylogenetic groups; we named these groups the anginosus group, the mitis group, the salivarius group, the bovis group, and the mutans group. S. mitis and S. gordonii clustered in the mitis group together with Streptococcus pneumoniae, Streptococcus oralis, Streptococcus sanguis, and Streptococcus parasanguis at levels of sequence homology of more than 96%. Within this group, S. mitis, S. oralis, and S. pneumoniae exhibited more than 99% sequence homology with each other, although the DNA-DNA similarity values for their total chromosome DNAs were less than 60%.