Structural studies were carried out on the O-polysaccharide fraction obtained from the lipopolysaccharide of Pseudomonas aeruginosa IID 1012, the standard strain of Homma serogroup K, by mild acid treatment. The O-polysaccharide was composed of L-rhamnose, N-acetyl-D-quinovosamine, and N-acetyl-D-galactosaminuronic acid. The results from analysis of fragments obtained by acid hydrolysis and Smith degradation of the O-polysaccharide, together with data on methylation analysis and nuclear magnetic reso-nance spectroscopic measurement of the polysaccharide, led to the most likely structure of the repeating units of the polymer chain, →4)D-GalNAcA(α1→3)D-QuiNAc(β1→2)L-Rha(αl→3)L-Rha(αl→, in which about 20% of the N-acetylgalactosaminuronic acid resi-dues were in an amide form and about 75% of the same residues were O-acetylated at C-3.