The structures of the O-antigenic polysaccharide chains of lipopolysaccharides of a number of Pseudomonas solanacearum strains were elucidated mainly with the help of methylation analysis and 13C NMR spectroscopy, including a computer-assisted 13C NMR-based analysis. Six structurally distinct but related polysaccharides were identified. They have a backbone which is built up of three L-rhamnopyranosyl residues and one 2-acetamido-2-deoxy-D-glucopyranosyl residue, and is unsubstituted or substituted with a residue of L-xylopyranose or L-rhamnopyranose as a monosaccharide side chain. The lipopolysaccharides of most of the strains contain polysaccharide chains consisting of at least two structurally different types of repeating units. Three of the polysaccharides are common to more than one strain.