The O-specific polysaccharide of the marine bacterium "Alteromonas marinoglutinosa" NCIMB 1770 was obtained by mild acid degradation of the corresponding lipopolysaccharide and found to contain D-galactose, N-acetyl-D-glucosamine, and N-acetyl-D-mannosamine residues in equimolar ratio. Based on methylation analysis, periodate oxidation, and 13C-NMR spectroscopy data of native and modified polysaccharides, the following structure of the trisaccharide repeating unit of the O-specific polysaccharide was established: [structure: see text]