AbstractRhizobium sp. SIN-1, isolated in India from root nodules on the tropical legume Sesbania aculeata, also induces nitrogen-fixing nodules on roots of S. macrocarpa, S. speciosa, S. procumbens, S. punicea, S. rostrata, and Vigna unguiculata. Unlike Azorhizobium caulinodans, SIN-1 does not induce stem nodules on S. rostrata. The nodules induced by SIN-1 develop exclusively at the bases of secondary roots. Electron microscopic studies of mature nodule sections revealed rhizobia within intercellular spaces, indicating a ‘crack entry’ mechanism of root infection. SIN-1 is a fast-growing, acid-producing, salt-tolerant Rhizobium that utilizes a wide variety of carbon sources. The nodulation (nod) genes of this strain are located on a 300-MDa symbiosis (sym) plasmid. Fatty acid profile and sequence comparison of a 260-bp conserved region of the 16S rRNA gene demonstrated that SIN-1 is phylogenetically closely related to R. galegae, a species that nodulates temperate legumes.