Xanthomonas hortorum pv. vitians is a Gram-negative bacterium that acts as the causative agent of bacterial leaf spot and headrot in lettuce. The lipopolysaccharide (LPS) of this bacterium is suspected to be an important molecule for adhesion to the plants. We have isolated the LPS, prepared the lipid A and the polysaccharide moieties thereof, and characterised all preparations by compositional analysis. Main sugar components are rhamnose and 3-acetamido-3,6-dideoxy-galactose which presumably furnish the O-specific polysaccharide. Other sugars are mannose, glucose, 6-deoxygalactose (fucose), and galacturonic acid, which should be core region constituents, and glucosamine, which builds up the carbohydrate backbone of lipid A. The LPS contains several phosphate groups, most of which are present in the core region. The main fatty acids in the lipid A are C10:0, 3-OH-C10:0 and 3-OH-C12:0. The latter is the only amide-linked fatty acid. Two fatty acids present in small amounts were identified, C8:0 and C11:0.