Studies by sugar and methylation analyses, Smith degradation, and 1H and 13C NMR spectroscopy revealed a structural heterogeneity in the O-polysaccharides of Pseudomonas syringae pvs. coronafaciens IMV 9030 and atrofaciens IMV 8281 owing to the presence of different types of repeating units. In strain IMV 9030, the major repeating units are a linear alpha-L-rhamnose trisaccharide and a tetrasaccharide (A, n=0 or 1). A minor repeating unit is a branched pentasaccharide with an alpha-L-rhamnose main chain and a lateral 3-acetamido-3,6-dideoxy-D-galactose (D-Fuc3NAc) residue (B, X=2, n=1). In strain IMV 8281, all repeating units are branched and differ in size and position of substitution of one of the alpha-L-rhamnose residues (tetrasaccharide, B, X=3, n=0; pentasaccharides, B, X=2 or 3, n=1). [structure--see text] Reinvestigation of the structure of the branched O-polysaccharide of P. syringae pv. tomato IPGR 140 showed that, together with the major tetrasaccharide repeating unit (B, X=3, n=0) [Knirel, Y. A., et al. Carbohydr. Res. 1993, 243, 199-204], it has a minor pentasaccharide repeating unit (B, X=3, n=1).