Xanthomonas campestris pv. vitians is a Gram-negative plant-associated bacterium that acts as causative agent of bacterial leaf spot and headrot in lettuce. The lipopolysaccharide 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 field NMR and MALDI-TOF MS. It consists of a nonrepetitive branched polysaccharide with a rhamnan backbone to which Fuc3NAc is linked. The NMR and MS approach led to the characterization of the fine structure of the polymer, which is randomly assembled. The rhamnan backbone is built up of beta-Rhap and alpha-Rhap, this last is present in one, two or three adjacent units and branched by an alpha-Fucp3NAc unit. This is a real case of a random constituted O-specific chain, therefore biosynthetic studies towards the comprehension of this irregular biosynthesis are needed.