The four families Nitrobacteraceae, Hyphomicrobiaceae, Phyllobacteriaceae, and Rhizobiaceae contain at least several genera of nitrogen-fixing, legume-nodulating, microsymbiotic bacteria.

Species of the Methylocystaceae are methanotrophs; they use methanol (CH3OH) or methane (CH4) as their sole energy and carbon sources.

The following genus has not been assigned to a family: These taxa have been published, but have not been validated according to the Bacteriological Code: The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature[7] and the phylogeny is based on whole-genome sequences.

[2][a] Parvibaculaceae Hyphomicrobiaceae Amorphaceae Rhodobiaceae Afifellaceae Cohaesibacteraceae Breoghaniaceae Stappiaceae Devosiaceae Aurantimonadaceae Ahrensiaceae Notoacmeibacteraceae Phyllobacteriaceae Rhizobiaceae Bartonellaceae Phyllobacterium [b] Brucellaceae Tepidamorphaceae Kaistiaceae Pseudoxanthobacteraceae Prosthecomicrobium [c] Pleomorphomonadaceae Blastochloridaceae Xanthobacteraceae Phreatobacteraceae Nitrobacteraceae Beijerinckiaceae Roseiarcaceae Methylocystaceae Chelatococcaceae Boseaceae Salinarimonadaceae Methylobacteriaceae Rhodobacterales Parvularculales Caulobacterales Natural genetic transformation has been reported in at least four Hyphomicrobiales species: Agrobacterium tumefaciens,[8] Methylobacterium organophilum,[9] Ensifer adhaerens,[10] and Bradyrhizobium japonicum.

[11] Natural genetic transformation is a sexual process involving DNA transfer from one bacterial cell to another through the intervening medium, and the integration of the donor sequence into the recipient genome by homologous recombination.