A sulfated O-specific polysaccharide containing D-mannose, L-rhamnose, and the sulfate group was obtained by mild acid hydrolysis of lipopolysaccharide (S-form) of the marine bacterium Pseudoalteromonas marinoglutinosa KMM 232. Based on analysis of methylation and 13C-NMR spectroscopy of native and desulfated polysaccharides, the following structure of disaccharide repeat unit in the O-specific polysaccharide has been established: [scheme]. This is the first report of a sulfated O-specific polysaccharide isolated from gram-negative bacteria.