A novel O-polysaccharide consisting of D-Xylp and L-Rhap in the molar ratio of 1:2.5 was identified as the major component in the lipopolysaccharide fraction of Xanthomonas campestris strain 642, which is responsible for a new bacterial disease of the strawberry plant. Its structure was mainly determined using chemical analysis, Smith degradation and 1D and 2D NMR spectroscopy experiments as: carbohydrate sequence [see text].