The O-polysaccharide of Providencia stuartii O33 was obtained by mild acid degradation of the lipopolysaccharide and the following structure of the tetrasaccharide repeating unit was established: -->6)-alpha-D-GlcpNAc-(1-->4)-alpha-D-GalpA-(1-->3)-alpha-D-GlcpNAc-(1-->3)-beta-D-Quip4N(Ac-D-Asp)-(1-->, where d-Qui4N(Ac-D-Asp) is 4-(N-acetyl-D-aspart-4-yl)amino-4,6-dideoxy-D-glucose. Structural studies were performed using sugar and methylation analyses and NMR spectroscopy, including conventional 2D 1H, 1H COSY, TOCSY, NOESY and 1H, 13C HSQC experiments as well as COSY and NOESY experiments in an H2O-D2O mixture to reveal correlations for NH protons. The O-polysaccharide of P. stuartii O33 shares an alpha-D-GlcpNAc-(1-->3)-beta-D-Quip4N(Ac-D-Asp) epitope with that of Proteus mirabilis O38, which seems to be responsible for a marked serological cross-reactivity of anti-P. stuartii O33 serum with the lipopolysaccharide of the latter bacterium. P. stuartii O33 is serologically related also to P. stuartii O4, whose O-polysaccharide contains a lateral beta-D-Qui4N(Ac-L-Asp) residue.