The O-polysaccharide was isolated from the lipopolysaccharide of Escherichia coli O168 and studied by chemical analyses and Smith degradation along with (1)H and (13)C NMR spectroscopies. The following structure of the branched pentasaccharide repeating unit of the O-polysaccharide was established: [carbohydrate structure: see text] where 6-O-acetylation of GlcNAc is partial. Reinvestigation of the O-polysaccharide of Shigella dysenteriae type 4 established earlier showed it to have the same structure except for that the lateral Fuc residue is nonstoichiometrically O-acetylated at each position.