The O-polysaccharide was isolated from the lipopolysaccharide of the plant-growth-promoting bacterium Azospirillum irakense KBCl and studied by sugar and methylation analyses, Smith degradation and H-1 and C-13 NMR spectroscopy, including H-1,C-13 HSQC and NOESY experiments for linkage and sequence analysis. The following structure of the branched hexasaccharide repeating unit of the O-polysaccharide with an unusually long side chain was established: alpha-D-Galf-(1-->2)-alpha-L-Rhap-(1-->3)-beta-D-Manp-(1-->3)-alpha-L-Rhap-(1-->3) -->4)-alpha-L-Rhap-(1-->3)-beta-D-Galp-(1--> (C) 2004 Elsevier Ltd. All rights reserved.