Occurrence of 2-keto-3-deoxyoctonate (KDO) in lipopolysaccharides (LPS) of genus Bacteroides (some strains have recently been reclassified as Porphyromonas or Prevotella) was examined. Strong-acid treatment of LPS isolated from Bacteroides fragilis, Bacteroides (Porphyromonas) gingivalis and Bacteroides intermedius (Prevotella intermedia) released periodate/thiobarbituric acid reaction-positive substances that were not detectable under conventional hydrolysis conditions. These substances were demonstrated to be KDO phosphate by high voltage paper electrophoresis before and after alkaline phosphatase treatment. KDO phosphate was also identified in these LPS by gas-liquid chromatography and gas-liquid chromatography/mass spectrometry. KDO was identified as well in both mild and strong-acid hydrolysates of LPS isolated from Bacteroides melaninogenicus (Prevotella melaninogenica). Neither KDO nor KDO phosphate was detectable in LPS of Bacteroides asaccharolyticus (Porphyromonas asaccharolytica) even after the strong-acid treatment of LPS. These findings indicate that there are possible structural variations in the inner core region of Bacteroides LPS.