The genus Fusobacterium currently includes 13 species. Fusobacterium nucleatum, the most frequently encountered species in humans, is heterogeneous and currently includes 5 subspecies. A potentially new subspecies of F. nucleatum that is intrinsically quinolone-resistant and phylogenetically separate from the other 5 subspecies has been identified from dog and cat oral flora. Two subspecies have been described for Fusobacterium necrophorum, and a new species, Fusobacterium equinum, which is related to F. necrophorum, has been described from horse oral flora. Additional molecular studies have characterized Fusobacterium ulcerans as separate from the phenotypically similar Fusobacterium mortiferum and Fusobacterium varium. Fusobacterium sulci and Fusobacterium alocis have been reclassified as Eubacterium sulci and Filifactor alocis, respectively. Fusobacterium prausnitzii is phylogenetically related to the Eubacterium-like organisms and will likely be reclassified in the future. The status of the remaining species is unchanged.