0 N E member of the genus Bacteroides-Bacteroides melaninogenicus-char- acteristically produces a jet-black pigment when grown on blood agar. This property was described by Oliver and Wherry (1921) in the original account of the organism, and has since been regarded as highly distinctive. The work described in this paper is based on the chance observation that other members of the genus are also capable of pigment production. Originally the black pigment of B. melaninogenicus was believed to be melanin (Oliver and Wherry). Subsequently the nature of the pigment was investigated by Schwabacher, Lucas and Rimington (1947). They identified it as haemin, but their cultures were grown on blood agar. Since pigment pro- duction by other members of the genus has now been recognised, a study of the pigment produced by these bacteria as well as that of B. melaninogenicus seemed to be of interest. MATERIALS AND METHODS Throughout this study, cellular morphology was checked routinely by Gram staining, and after incubation the purity of the cultures was tested by subculture on blood agar. Before the investigation was commenced the fermentation reactions of the strains employed were studied, using the medium of Reed and Orr (1941). Organisms. The following strains were obtained from the National Collection of Type Cultures : Bacteroides melaninogenicus (NCTC9336), B. fragilis (NCTC9343) and B. necro- phorus (NCTC7155). Partially defined media were supplemented with sterile ox serum 4 ml per cent. (v/v) and haemin 0.001 per cent. (w/v) for the cultivation of B. melaninogenicus. Sterilisation was by autoclaving at 15 lb. per sq. in. (121°C) for 20 min., unless otherwise stated. The serum was sterilised by Seitz filtration. The composition of the partially defined medium is shown in table I. Solution A was first prepared and dispensed in 100-ml quantities. After sterilisation it could be stored for a maximum period of 1 mth. The ingredients of solution B were dissolved in the solvents recommended by Meynell and Meynell (1965), with aseptic precautions. The concentration of the biotin solution was 0.1 pg per ml and of the cyanocobalamin