根瘤菌目是变形菌门中的一目，其中有许多科有会固氮作用且与植物根部共生的根瘤菌。另外，布鲁氏菌科和巴通体科则有些会使人致病的物种，这些疾病分别称为布鲁氏症和巴通体症。

Rhizobiales is an order of alpha proteobacteria. They are Gram-negative.

The rhizobia, which fix nitrogen and are symbiotic with plant roots, appear in several different families. The four families Bradyrhizobiaceae, Hyphomicrobiaceae, Phyllobacteriaceae, and Rhizobiaceae contain at least six genera of nitrogen-fixing, legume-nodulating, microsymbiotic bacteria. Examples are the genera Bradyrhizobium and Rhizobium. Species of Methylocystaceae are methanotrophs; they use methanol (CH₃OH) or methane (CH₄) as their sole energy and carbon sources. Other important genera are Bartonella (pathogen) and Agrobacterium (genetic engineering).