The capsular polysaccharide of the pneumococcal type XVIIIA consists of d-galactose, Dglucose, rhamnose, N-acetylglucosamine, glycerol, and phosphate in the approximate ratios 2:3.5:1:1:1:1. Its behavior on oxidation with periodate and some of its serological reactivities resemble those of the O-deacetylated polysaccharide of pneumococcal type XVIII. The residues of galactose and rhamnose are probably linked 1,3-, and the N-acetylglucosamine is also stable to periodate. © 1964, American Chemical Society. All rights reserved.