The characteristics of the fusiform species Bacteroides forsythus, isolated from human periodontal pockets, were examined. 165 rDNA sequence analysis confirmed that B. forsythus was not a species within the genus Bacteroides sensu stricto. Although B. forsythus was phylogenetically related to Bacteroides distasonis and Bacteroides merdae in the phylogenetic tree, the ratio of anteiso-15:0 to iso-15:0 in whole-cell methanolysates of B. forsythus was different from those of B. distasonis, B. merdae and other Bacteroides species. B. forsythus did not grow on medium containing 20% bile, but members of the Bacteroides fragilis group did. B. forsythus was the only species tested that was trypsin-positive in API ZYM tests. The dehydrogenase enzyme pattern was of no use for the differentiation of B. forsythus and the B. fragilis group. On the basis of these data, a new genus, Tannerella, is proposed for Bacteroides forsythus, with one species, Tannerella forsythensis corrig., gen. nov., comb. nov. The type strain of Tannerella forsythensis is JCM 10827T (= ATCC 43037T).