Capsular polysaccharides (CPSs) from six representative strains of Acetobacter diazotrophicus were isolated and fractionated by gel filtration and anion-exchange chromatography. Purified CPSs obtained in the non-adsorbed fraction of a DEAE-Sephadex A-25 column were qualitatively and quantitatively analyzed for sugar composition. Uronic acid and amino sugars were not detected in all purified CPSs. Basically the CPSs of A. diazotrophicus are composed of rhamnose, mannose, galactose and glucose. The presence of fucose was only observed in the CPS of strains PR2 and PAL3. Based on these results, the six strains of A. diazotrophicus could be divided into four groups according to the sugar content of their capsules: (i) fucose-containing capsules (PR2 and PAL3, localized in roots), (ii) mannose-rich capsule (PAL5, localized in root), (iii) capsules with a high ratio of hexose to rhamnose (PR4 and PR20, localized in stems) and (iv) capsules with a low ratio of hexose to rhamnose (PR14, localized in rhizosphere). For all CPSs, sodium dodecy sulfate-polyacrylamide gel electrophoresis showed diffuse bands of slow mobility in silver-stained gels. The different CPS migration patterns could not be correlated with sugar composition. The purified CPS of strain PAL3 was found to be immunogenic and immunochemically similar to the CPS of strain PR2. The serological specificity to CPS of strains PAL3 and PR2 correlated well with the presence of focuse, indicating that this deoxyhexose is immunodominant.(ABSTRACT TRUNCATED AT 250 WORDS)