Acid O-specific polysaccharide containing D-glucose, D-glucuronic acid, L-fucose, and 2-acetamido-2-deoxy-D-glucose was obtained by mild acid degradation of lipopolysaccharide from Providencia alcalifaciens O46. Consideration of the data revealed the following structure of the hexasaccharide repeating unit of O-specific polysaccharide under methylation analyses along with (1)H and (13)C NMR spectroscopy, including 2D (1)H, (1)H-COSY, TOCSY-, ROESY-,(1)H, (13)C-HSQC-, and HMQC-TOCSY experiments: [Formula: see text].