The structure of an acidic O-specific polysaccharide from the marine bacterium Cellulophaga baltica was established by chemical methods and NMR spectroscopy. The polysaccharide was shown to consist of repeating tetrasaccharide units containing two mannose residues, one N-acetyl-D-glucosamine residue, and one D-glucuronic acid residue. An O-acetyl group was also found in the polysaccharide in nonstoichiometric amount. The polysaccharide had the following structure: [graphic not available: see fulltext]