An O antigenic polymer containing L-rhamnose, D-mannose, and L-xylose was isolated from the lipopolysaccharide present in the reference strain for Stenotrophomonas (Xanthomonas or Pseudomonas) maltophilia serogroup O2, by mild acid hydrolysis and gel-permeation chromatography. By means of NMR spectroscopy and chemical degradations, the polysaccharide was found to be based on a branched trisaccharide repeating-unit of the structure shown. [formula: see text].