Partial hydrolysis with acid, methylation analysis (including uronic acid degradation), Smith degradation, and p.m.r. spectroscopy have been used to determine the primary structure of the capsular polysaccharide of Klebsiella serotype K64. The hexasaccharide repeating-unit, which also contains one O-acetyl substituent, comprises a 4)-alpha-D-GlcpA-(1 leads to 3)-alpha-D-Manp-(1 leads to 3)-beta-D-Glcp-(1 leads to 4)-alpha-D-Manp-(1 leads to chain with a 4,6-O-(1-carboxyethylidene)-beta-D-glucopyranosyl and an L-rhamnosyl group attached to the 4-linked D-mannosyl residue at 0-2 and 0-3, respectively.