O-specific polysaccharide (O-antigen) of the lipopolysaccharide of Proteus penneri 20 was studied using sugar analysis along with various one- and two-dimensional NMR spectroscopy techniques. The structure of the polysaccharide was established. It has the same carbohydrate backbone structure as that described earlier for P. penneri 16, in which the positions of the O-acetyl groups have not been determined. P. penneri 20 O-antiserum showed a strong cross-reactivity with the lipopolysaccharides of P. penneri 10, 16, 18, 32, 45 and P. mirabilis O17. These data enable classifying these strains together with P. penneri 20 in one Proteus serogroup, O17.