Streptococci that produced acetoin and alkaline phosphatase, hydrolyzed arginine, and fermented trehalose but did not produce pyrrolidonylarylamidase or β-glucuronidase, split hippurate, or ferment ribose or glycogen were collected and compared. These streptococci were considered members of the unapproved species “Streptococcus milleri,” but most of them would fit the description of one of three approved species: Streptococcus anginosus, Streptococcus constellatus, or Streptococcus intermedius. Most were recent clinical isolates. Some hydrolyzed esculin and fermented lactose, while others did not. Some fermented mannitol and raffinose. Many were beta-hemolytic, and several reacted to antiserum of Lancefield group A, C, F, or G. From a total of 111 strains, 40 were selected for comparison of their deoxyribonucleic acid (DNA) base sequence similarities by DNA-DNA hybridization on membrane filters. All biotypes, hemolytic types, and serotypes were included, as well as the type strains of S. anginosus, S. constellatus, and S. intermedius, and Lancefield group F Streptococcus sp., plus two strains derived from Guthof's “S. milleri” isolates. The results showed considerable genetic similarity within the group. DNA from most strains hybridized at a level of 70% or more of the homologous control, even under very stringent conditions. There was somewhat less homology between DNAs of some of the least reactive strains (lactose, mannitol, and esculin negative) and the most reactive strains (lactose, mannitol, and esculin positive). The Lancefield F strain and the type strains of S. anginosus and S. constellatus were genetically similar. Traits such as hemolysis and lactose fermentation could not be ascribed to plasmids. The results support the unification of these streptococci into a single species.