An acidic polysaccharide from Alteromonas macleodii 2MM6 is shown to consist of tetrasaccharide repeating units containing D-galactose, 3-O-acetyl-2-acetamido-2-deoxy-D-glucose, 2-acetamido-2-deoxy-L-guluronic acid and 3,6-dideoxy-3-(4-hydroxybutyramido)-D-galactose residues. On the basis of sugar analysis, 1H and 13C NMR-spectroscopy data including NOE and one-dimensional HOHAHA experiments, the following structure was suggested for the polysaccharide repeating unit: [formula: see text] R: -CO-CH2-CH2-CH2-OH.