The nodulation of legumes has for more than a century been considered an exclusive capacity of a group of microorganisms commonly known as rhizobia and belonging to the ␣-Proteobacteria. However, in the last 3 years four nonrhizobial species, belonging to ␣ and  subclasses of the Proteobacteria, have been described as legume-nodulating bacteria. In the present study, two fast-growing strains, LUP21 and LUP23, were isolated from nodules of Lupinus honoratus. The phylogenetic analysis based on the 16S and 23S rRNA gene sequences showed that the isolates belong to the genus Ochrobactrum. The strains were able to reinfect Lupinus plants. A plasmid profile analysis showed the presence of three plasmids. The nodD and nifH genes were located on these plasmids, and their sequences were obtained. These sequences showed a close resemblance to the nodD and nifH genes of rhizobial species, suggesting that the nodD and nifH genes carried by strain LUP21 T were acquired by horizontal gene transfer. A polyphasic study including phenotypic, chemotaxonomic, and molecular features of the strains isolated in this study showed that they belong to a new species of the genus Ochrobactrum for which we propose the name Ochrobactrum lupini sp. nov. Strain LUP21 T (LMG 20667 T ) is the type strain.Plants from the family Leguminosae are usually capable of dinitrogen fixation because of their symbiotic interaction with nodulating bacteria belonging to the order Rhizobiales. Most bacteria that establish a symbiosis with legume plants, including some nonrhizobial species of Methylobacterium (16, 45) and Devosia (40, 41), belong to the ␣ subclass of Proteobacteria, although some species from genera of the  subclass, such as Ralstonia and Burkholderia, can also nodulate legumes (6,32,53). In the last few years there has been an increasing amount of research focused on bacteria that nodulate stems or roots of tropical legume species. However, the identity of many of the endosymbionts of temperate legumes still remains unknown. The genus Lupinus groups up to 200 species of herbs and small shrubs, broadly distributed in the Mediterranean area and in the American continent, where they colonize very different environments. Despite the agronomic and ecological interest of Lupinus, this plant has been poorly studied with respect to its symbionts. Plants from the genus Lupinus are nodulated by fast-and slow-growing rhizobia; however, slow-growing rhizobia are more frequently isolated from this legume (3,5,21,30). The data obtained from the small-subunit (SSU) rRNA gene indicate a very close relationship between some bradyrhizobia isolated from Lupinus and Bradyrhizobium japonicum (3,12,30). However, bacterial strains nodulating Lupinus plants have been poorly characterized thus far, and fast-growing species nodulating this legume have not been not officially described; nevertheless, in the past the species Rhizobium lupini was proposed (15) and was later abandoned (14).During a study of rhizobia nodulating Lupinus plants in several geographical regi...