The family Desulfurellaceae is part of the class Deltaproteobacteria in the phylum Proteobacteria and contains the genera Desulfurella and Hippea. There are seven valid species in the family which are all strictly anaerobic and moderately thermophilic bacteria. Members of the family are mostly obligately sulfur respiring, though Desulfurella propionica can use thiosulfate as an electron acceptor besides elemental sulfur. During growth, organic substrates are completely oxidized with CO2 and H2S as products. Lithotrophic growth with hydrogen as an electron donor is possible by all species except Desulfurella acetivorans. The genomes of D. acetivorans, Hippea maritima, Hippea alviniae, and Hippea sp. strain KM1 have been sequenced and are all slightly smaller than 2 mega base pairs in length. Members of the family are found mostly in warm-hot sulfur-containing anoxic environments, such as hydrothermal springs and sediments, and often in association with microbial mats. The DNA base composition (mol% G+C) is typically low for family members and is a distinguishing characteristic between the two genera, Desulfurella (31-33 mol%) and Hippea (35-40 mol%). While officially classified as part of Deltaproteobacteria, the family is more closely related phylogenetically to members of the class Epsilonproteobacteria.