The group-specific polysaccharide of the group B Streptococcus was isolated by nitrous acid extraction followed by gel filtration on Sepharose 6B and chromatography on DEAE-Bio-Gel A. It was composed of rhamnose, galactose, N-acetylglucosamine, and glucitol phosphate. Mild periodate oxidation of the polysaccharide resulted in a rapid reduction in molecular weight, indicating that the glucitol was located in the backbone of the polymer. High-resolution 31P NMR showed the presence of a single type of phosphodiester bond in the molecule. Methylation analysis and several specific chemical degradations were done to determine sugar linkages. The basic structure of the group B polysaccharide consists of a backbone of 2-linked rhamnose, 2,4-linked rhamnose, and glucitol phosphate, and side chains of rhamnose(1 → 3)galactose(1 → 3)N-acetylglucosamine linked to the 4-position of a rhamnose in the backbone. © 1984.