The cellular morphology, colonial morphology, biochemical properties, DNA base compositions, and DNA-DNA homolgies of three biovars of Fusobacterium necrophorum were examined. Some differences were found among the three biovars in cellular morphology, colonial morphology, and biochemical properties. The guanine-plus-cytosine contents of DNAs from biovar C strains Fn521T (T = type strain), Fn522, and Fn520 were 30.4, 29.3, and 28.0 mol%, respectively, and the guanine-plus-cytosine contents of DNAs from strains VPI 2891 (biovar A) and VPI 6161 (biovar B) were 31.3 and 32.0 mol%, respectively. Labeled DNA from biovar C strain Fn521T exhibited 96 and 82% relatedness to DNAs from biovar C strains Fn522 and Fn520, respectively; however, it exhibited only about 10% relatedness to DNAs from strains of biovars A and B. Labeled DNAs from strains VPI 2891 and VPI 6161 exhibited more than 70% relatedness to each other, but about 6 to 20% relatedness to DNAs from biovar C strains. Therefore, Fusobacterium pseudonecrophorum sp. nov., nom. rev. (ex Prévot 1940) is proposed for Fusobacterium necrophorum biovar C. The type strain is strain Fn521 (= JCM 3722).