The capsule of Bacteroides fragilis (ATCC23745) consists of two distinct polysaccharides, the separation of which could not be accomplished. The mouse-passaged strain (23745M1), however, yielded a preponderant polysaccharide which was isolated and purified. Using mainly high resolution NMR spectroscopy, the structure of the polysaccharide was elucidated and it is composed of the following repeating unit:where α-l-d-Yer is 3,6 dideoxy-4-C-(l-glycero-4 -hydroxyethyl)-α-d-xylo-hexopyranoside.