DNA base composition, DNA-DNA homology and long-chain fatty acid studies were performed on Streptococcus thermophilus and Streptococcus salivarius. These species possess similar mol % G + C values (about 37 to 41), long-chain fatty acid profiles and belong to a single DNA homology group. On the basis of the present and earlier studies it is proposed that Streptococcus thermophilus (Orla-Jensen) be reclassified as Streptococcus salivarius subsp. thermophilus comb. nov.