Sinorhizobium morelense sp. nov. is described to designate a group of bacteria isolated from root nodules of Leucaena leucocephala. S. morelense shows 98% 16S rRNA gene sequence similarity to some Sinorhizobium species and to Ensifer adhaerens. This novel species is distinguished from other Sinorhizobium species and from E. adhaerens by DNA–DNA hybridization, 16S rRNA gene restriction fragments and sequence and some distinctive phenotypic features. Strains of this species are highly resistant to some antibiotics, such as carbenicillin (1 mg mlN1), kanamycin (500 lgmlN1) and erythromycin (300 lgmlN1). They do not form nodules, but a nodulating strain, Lc57, is closely related to the novel species. Strain Lc04T (=LMG 21331T= CFN E1007T) is designated as the type strain of this novel species.