The acidic capsular polysaccharide from the museum strain 2211 of Klebsiella ozaenae serotype K4 is built up of pentasaccharide repeating-units that contain residues of D-glucose, D-mannose, D-glucuronic acid, and 3-deoxy-D-glycero-D-galacto-nonulosonic acid (Kdn) in the ratios 2:1:1:1, as well as an O-acetyl group. The last-named sugar, which is reported in bacterial polysaccharides for the first time, was identified as the methyl (methyl 3-deoxynonulopyranosid)onate obtained by methanolysis of the polysaccharide. On the basis of the results of partial acid hydrolysis, Smith degradation, and computer-assisted 13C-n.m.r. analysis, it was concluded that the capsular polysaccharide has the following structure: (formula see text)