The O-polysaccharide was obtained by degradation of the lipopolysaccharide of Providencia alcalifaciens O2 under mild acidic conditions followed by GPC. The polysaccharide was found to contain two unusual components: 3,6-dideoxy-L-arabino-hexose (ascarylose, Asc) and 2-(L-alanyl)amino-2-deoxy-D-glucose (GlcNAla). Ascarylose was partially split off during lipopolysaccharide degradation and could be eliminated completely by selective acid hydrolysis, which also partially cleaved the β-GAlNAc-(1 → 6) linkage. The following structure of the branched pentasaccharide repeating unit was established by (1)H and (13)C NMR spectroscopy of the O-polysaccharide and O-deacetylated polysaccharide, as well as products of partial acid hydrolysis: α-Ascp-(1 → 4)-α-D-GlcpA-(1 → 4) → 6)-β-D-GlcpNAla-(1 → 4)-β-D-GlpA-(1 → 3)-β-D-GalpNAc-(1 → ~60% OAc--3).