Isolates of Fusobacterium that differ from type strains of various fusobacterial species with respect to DNA sequence, cellular fatty acid composition, and biochemical activity, were obtained from periodontitis lesions in a patient with insulin-dependent diabetes mellitus. These isolates have the following distinguishing characteristics: 28% guanine + cytosine content; 40% or less DNA homology with type strains of representative fusobacterial species; cell size, 0.5 - 1 X 4 -100 microns; absence of motility; ability to ferment glucose, fructose, and galactose, but not 25 other carbohydrates; ability to produce indole; ability to hydrolyze hippurate but not esculin; sensitivity to bile; ability to produce little or no gas; ability to utilize threonine but not lactate. We propose that the organisms be classified as a distinct species of Fusobacterium to be named Fusobacterium periodonticum. The type strain of this new species has been deposited with the American Type Culture Collection under the designation ATCC 33693.