Reduced minus oxidized difference spectra of 9 different species of anaerobic gramnegative bacteria were examined spectroscopically. The cytochrome b-type spectra, with an a-absorption band at 565 nm and a Soret band at near 433 nm, were determined in all of the following bacteria, Bacteroides oralis, Bacteroides asaccharolyticus, Bacteroides intermedius, Bacteroides loescheii, Bacteroides fragilis, Capnocytophaga ochracea, Fusobacterium nucleatum, Veillonella parvula and Veillonella atypica. In B. oralis, a b-type cytochrome and a CO-binding pigment which may be an o-type cytochrome were present in the membrane. However, the absence or minor presence of cytochrome c was observed in the membrane and soluble fractions. The b-type cytochrome of B. oralis was reduced by NADH, succinate and malate but not by fumarate. The reduction of the b-type cytochrome by NADH was inhibited by HOQNO and amytal.