The cellular fatty acids of 31 strains belonging to the Streptococcus milleri group were analysed by capillary gas-liquid chromatography. Results were compared with findings from biochemical differentiation of the strains into Streptococcus constellatus (two strains), Streptococcus anginosus (16 strains) and Streptococcus intermedius (13 strains). Eight strains of various other streptococci were included as internal references, including three strains of streptococcus morbillorum, three strains of beta-hemolytic streptococci, and two strains of enterococci. The Streptococcus milleri strains formed a very homogeneous group according to fatty acid composition and were easily differentiated from other groups. However, within the group, it was not possible to differentiate Streptococcus constellatus, Streptococcus anginosus and Streptococcus intermedius by fatty acid composition alone.