A. beijerinckii strain B-1615 produced two acidic exopolysaccharides in the ratio approximately 9:1. The minor polysaccharide contained mannuronic and guluronic acids in the ratio 2.3:1 and is a bacterial alginate. The major polysaccharide consisted of D-galactose, L-rhamnose, and pyruvic acid in the ratios 2:1:1 and was acetylated. On the basis of methylation analysis, and 1H- and 13C-n.m.r. spectroscopy of the polysaccharide before and after removal of the pyruvic acid residues and O-deacetylation, it was concluded that the major polysaccharide had the structure [formula; see text] with up to 1.5 OAc groups per repeating unit.