The complete primary structure of the capsular polysaccharide of Clostridium perfringens Hobbs 5, an anaerobic bacterium implicated in food poisoning, was determined. The polysaccharide was isolated from C. perfringens Hobbs 5 cells, after deproteination, by ethanol precipitation and by ion-exchange chromatography. The polysaccharide was comprised of glucose, galactose, mannose, N-acetylglucosamine, N-acetylgalactosamine, and glucuronic acid, in equimolar ratios. Sequence and linkage assignments of the glycosyl residues were obtained by NMR spectroscopy, specifically by the combination of two-dimensional homonuclear TOCSY and NOESY experiments and heteronuclear (1H, 13C) multiple-quantum coherence (HMQC, HMQC-COSY, HMQC-TOCSY and HMBC) experiments. Thus, the envelope polysaccharide of C. perfringens Hobbs 5 was found to be a polymer composed of a hexasaccharide repeating unit with the following structure: [formula: see text] This structure is novel among bacterial cell-surface polysaccharides, and it is the first of many serotypically distinct capsular polysaccharides of C. perfringens to be described.