The O-polysaccharide (O-antigen) was obtained from the lipopolysaccharide of Proteus myxofaciens,aProteus strain producing copious amounts of slime, which was isolated from the gypsy moth larvae. The structure of the polysaccharide was studied by chemical analysis and 1 H and 13 C NMR spectroscopy, including 2D COSY, TOCSY, ROESY and H-detected 1 H, 13 CHMQC experiments.