Xanthomonas is a Gram-negative plant-associated bacterium that acts as causative agent of bacterial leaf spot and headrot in lettuce. The lipopolysac-charide of this bacterium is suspected to be an important molecule for adhesion to and infection of the plants. The lipopolysaccharide has been isolated from the phenol phase and the O-specific chain characterized by compositional analysis, high fieldNMRandMALDI-TOFMS.