Heterogeneity among isolates of Fusobacterium nucleatum has been recognized for many years. The phenotypic properties of 340 strains considered to be F. nucleatum were examined. While these strains were phenotypically similar and fit the description of F. nucleatum, they could be differentiated into three groups on the basis of electrophoretic patterns of whole-cell proteins and DNA homology. Strains in groups I and II showed >80% DNA homology within groups and 85% DNA homology to each other and Fusobacterium nucleatum be divided into the following three subspecies: Fusobacterium nucleatum subsp. nucleatum, with type strain ATCC 25586; Fusobacterium nucleatum subsp. polymorphum, with type strain ATCC 10953; and Fusobacterium nucleatum subsp. vincentii, with type strain ATCC 49256.