Structural studies have been carried out on the O-specific polysaccharide, the core oligosaccharide, and the lipid A from the lipopolysaccharide of Pseudomonas maltophilia NCTC 10257. By means of 13C nuclear magnetic resonance spectroscopy, a tetrasaccharide repeating-unit for the O-specific polymer has been confirmed. However, the data suggest that the L-rhamnopyranosyl residue at the branching point has the alpha configuration rather than beta as proposed previously [Neal, D.J. and Wilkinson, S.G. (1979) Carbohydr. Res. 69, 191-201]. The core oligosaccharide contains residues of D-glucose, D-mannose phosphate, D-galactosamine (not N-acetylated), D-galacturonic acid, and a 3-deoxyoctulosonic acid (but no aldoheptose). A partial structure for the oligosaccharide is proposed. Lipid A is based on phosphorylated glucosamine residues, with N-fatty acyl and O-fatty acyl substituents. The major fatty acids are 9-methyldecanoic acid, 2-hydroxy-9-methyldecanoic acid, 3-hydroxy-9-methyldecanoic acid (each ester-linked), 3-hydroxydodecanoic acid, and 3-hydroxy-11-methyldodecanoic acid (both mainly amide-linked). The results of this study provide further evidence for a relationship between P. maltophilia and some Xanthomonas species.