The Rhizobiaceae are a family of proteobacteria, including many species of rhizobia, as well as plant parasites such as Agrobacterium. The Rhizobiaceae are, like all Proteobacteria, gram-negative. They are aerobic, and the cells are usually rod-shaped. Many species of the Rhizobiaceae are diazotrophs which are able to fix nitrogen and are symbiotic with plant roots.