Abstract The acidic polysaccharide secreted by Rhizobium phaseoli strain 127 K38 was found to consist of a decasaccharide repeating-unit. The absolute configuration, ring form, points of attachment of each glycosyl residue, anomeric configurations of the glycosyl linkages, and sequence of the glycosyl residues were determined. Each of the glycosyl residues of the polysaccharide was determined to be in the D configuration and the pyranoid ring-form. The polysaccharide was found to contain a pyruvic acetal group in the ( S) configuration, and acetyl groups linked to at least two of the oxygen atoms. The decasaccharide repeating-unit is shown on the following page.