DNA fragments with ams genes of Erwinia amylovora and cps genes of Erwinia stewartii were transferred to exopolysaccharide (EPS)-deficient mutants of the other species. The resulting EPSs were characterized by sensitivity to EPS-dependent bacteriophages, staining with amylovoran-specific fluorescein-isothiocyanate-labelled lectin and chemical techniques, such as determination of the sugar composition and methylation analysis in order to distinguish between amylovoran and stewartan. Degradation by the stewartan-dependent phage phi-K9 was used to detect stewartan production, and staining with a lectin from Abrus precatorius detected amylovoran capsules. The patterns of sugar linkages were determined by methylation analysis. Stewartan contained a significantly higher glucose to galactose ratio than amylovoran and produced a characteristic signal from 6-linked glucose residues. By these criteria, most E. stewartii cps mutants displayed exclusively amylovoran synthesis when complemented with the E. stewartii cps genes produced stewartan. The complementation to an EPS-positive phenotype may require most genes of the ams or the cps operon. An exception was an E. stewartii cpsK mutant that made predominantly stewartan when complemented with the ams cosmid. IR spectra showed that both amylovoran and stewartan were acylated when synthetized in E. amylovora, but not in E. stewartii. The amylovoran-producing E. stewartii merodiploids regained virulence to corn seedlings when mucoidy was restored by the ams cluster, but the stewartan-producing E. amylovora ams-/cps+ strains were weakly virulent on pear slices and avirulent on apple seedlings.