Sesbania spp. can establish symbiotic interactions with rhizobia from diverse taxonomical origins, including the newly described Sinorhizobium saheli and S. teranga bv. sesbaniae, in addition to the Sesbania rostrata-specific symbiont Azorhizobium caulinodans. These different rhizobia exhibit a narrow host range, which is limited mainly to Sesbania spp. Nod factors from overproducing strains of S. saheli ORS611 were purified and their structures determined. Remarkably, the terminal reducing glucosamine of most compounds was found to bear both an arabinosyl group on C-3 and a fucosyl substitution on C-6. Other structural features are as follows: Nod factors are mainly chitopentameric compounds, N-methylated, O-carbamoylated and N-substituted either by a C_18:1 or a C_16:0 acyl chain at their nonreducing end. Nod factors from an overproducing strain of S. teranga bv. sesbaniae ORS604 were found to be identical to those of S. saheli on the basis of high-pressure liquid chromatography separation and liquid secondary ion mass spectrometry analysis. The Nod factor double glycosylation, i.e., arabinosylation and fucosylation, which appears to be unique to Sesbania symbionts, probably reflects a high selection pressure from Sesbania host plants.