An extracellular acidic polysaccharide, elaborated by Bacillus polymyxa, was composed of D-glucose, D-mannose, Dgalactose, and D-glucuronic acid (approximate molar ratio of 4:3: 1:l). Methylation and fragmentation analysis by partial acid hydrolysis indicated that the polysaccharide had a complicated, highly branched structure, consisting mainly of P(1+ 3) and (1 + 4) D-glycosidic linkages. The backbone chain containing D-glucose, D-mannose, and D-glucuronic acid residues is attached at the C-4, C-3, and C-4positions, respectively, one out of every three glucose residues being substituted at the C-2 position to form two kinds of branches at the C-2 and C-4 positions, with side chains of single or a few carbohydrate units that are terminated by D-glucose and (or) D-mannose residues.