Peptidoglycans of organisms belonging to the strictly anaerobic family Bacteroidaceae were investigated for the presence of lanthionine. Different procedures for the quantitation of lanthionine were compared. Performic acid and peroxide oxidation procedures on 35S-labeled peptidoglycan from Fusobacterium nucleatum Fev1 resulted in low yields of cysteic acid (42 and 60%, respectively) and many other additional unidentified oxidation products. Lanthionine was, however, recovered in high yield (89% or more) from acid hydrolysates of unoxidized peptidoglycans. Lanthionine was found exclusively in some species of Fusobacterium, in particular F. nucleatum, F. necrophorum, F. russi, and F. gonidiaformans, for which lanthionine may be ascribed a function as a taxonomic marker. Peptidoglycans of these bacteria are thus proposed to belong to a new chemotype, assigned A1 delta. One strain of Fusobacterium, F. mortiferum VPI 0473 contained both lanthionine and diaminopimelic acid in about equal proportions. Species of F.plauti had a composition atypic of gram-negative cells. Chemotypic differences were also indicated among the species of Bacteroides investigated. Thus, some species contained lysine and not diaminopimelic acid as the major dibasic amino acid (e.g., F. asaccharolyticus). It is concluded that peptidoglycans of gram-negative organisms constitute a somewhat more heterogeneous group than hitherto assumed.