The virulence of different black‐pigmentedBacteroidesstrains (BPB) was studied using monoinfections in an experimental animal model. Bacterial suspensions were injected subcutaneously in the back of mice. After four days the mice were killed and the induced inflammations were examined histologically.Bacteroides gingivalisstrains, which are mostly isolated from subgingival plaque associated with adult destructive periodontitis. were characterized by inducing a spreading type of inflammation, mostly resulting in a gravity abscess or a phlegmonous abscess. Strains ofBacteroides melaranogenicussubspeciesintermedius, isolated from gingivitis or periodontitis, always induced a localized abscess at the site of injection. MostBacteroides asaccharolyticusstrains, which are isolated from nonoral sites, caused a localized inflammation, andBacteroides melaninogenicussubspeciesmelaninogenicusstrains caused only minimal inflammatio