The O-polysaccharide was obtained from the lipopolysaccharide of the stem-nodulating nitrogen-fixing bacterium Azorhizobium caulinodans HAMBI 216 and studied by sugar and methylation analyses along with (1)H and (13)C NMR spectroscopy. The polysaccharide was found to have a linear pentasaccharide repeating unit containing D-rhamnose and its rarely occurring 2-O-methyl (Rha2OMe) and 3-C-methyl (Rha3CMe) derivatives and having the following structure: →3)-α-D-Rhap2OMe-(1→2)-β-D-Rhap3CMe-(1→3)-α-D-Rhap-(1→2)-β-D-Rhap3CMe-(1→3)-α-D-Rhap-(1→ .