The partial 16S rRNA sequences of 24 Streptococcus species were determined by reverse transcription. A comparative analysis of these sequences and the sequences of seven previously studied streptococcal species revealed the presence of several clusters within the genus. The clusters obtained from the sequence analysis agreed in general with the groups outlined on the basis of the results of nucleic acid hybridization studies, but there were some exceptions. The pyogenic group was extended to include Streptococcus agalactiae, S. parauberis, S. porcinus, and S. uberis. Four oral groups were discerned; these four groups centered on S. mutans, S. salivarius, S. anginosus, and S. oralis. Some species (e.g., S. suis and S. acidominimus) did not cluster with any particular group. Our findings are discussed in the context of data from other genetic and chemotaxonomic studies.