Until recently, black-pigmented Gram-negative anaerobes were classified as 'black-pigmented Bacteroides'. At present, 11 distinct species are recognized in this group. Because of major differences with Bacteroides fragilis, the type species of the genus Bacteroides, new genera have been proposed: Porphyromonas for three asaccharolytic species, and Prevotella for the saccharolytic species. Typing methods have been developed for some species of black-pigmented Gram-negative anaerobes. These include biotyping and serotyping, but relatively few types can be distinguished with these methods. Recently, DNA restriction endonuclease analysis has been used for typing of P. gingivalis, Pr. intermedia and P. endodontalis strains. Great heterogeneity was observed within all three species. This typing method can be useful for epidemiological studies.