<jats:p>A novel thermophilic, chemoheterotrophic, Gram-negative-staining, multicellular filamentous bacterium, designated strain 110S<jats:sup>T</jats:sup>, was isolated from an iron-rich coastal hydrothermal field in Japan. The isolate is facultatively aerobic and chemoheterotrophic. Phylogenetic analysis using 16S rRNA gene sequences nested strain 110S<jats:sup>T</jats:sup> in a novel class-level clone cluster of the phylum ‘<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="phylum" xlink:type="simple"> <jats:ext-link ext-link-type="uri" xlink:href="http://dx.doi.org/10.1601/nm.550" xlink:type="simple"> <jats:italic>Chloroflexi</jats:italic> </jats:ext-link> </jats:named-content>’. The isolate grows by dissimilatory iron- and nitrate-reduction under anaerobic conditions, which is the first report of these abilities in the phylum ‘<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="phylum" xlink:type="simple"> <jats:ext-link ext-link-type="uri" xlink:href="http://dx.doi.org/10.1601/nm.550" xlink:type="simple"> <jats:italic>Chloroflexi</jats:italic> </jats:ext-link> </jats:named-content>’. The organism is capable of growth with oxygen, ferric iron and nitrate as a possible electron acceptor, has a wide range of growth temperatures, and tolerates higher NaCl concentrations for growth compared to the other isolates in the phylum. Using phenotypic and phylogenetic data, strain 110S<jats:sup>T</jats:sup> ( = JCM 17282<jats:sup>T</jats:sup> = NBRC 107679<jats:sup>T</jats:sup> = DSM 23922<jats:sup>T</jats:sup> = KCTC 23289<jats:sup>T</jats:sup> = ATCC BAA-2145<jats:sup>T</jats:sup>) is proposed as the type strain of a novel species in a new genus, <jats:italic>Ardenticatena maritima</jats:italic> gen. nov., sp. nov. In addition, as strain 110S<jats:sup>T</jats:sup> apparently constitutes a new class of the phylum ‘<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="phylum" xlink:type="simple"> <jats:ext-link ext-link-type="uri" xlink:href="http://dx.doi.org/10.1601/nm.550" xlink:type="simple"> <jats:italic>Chloroflexi</jats:italic> </jats:ext-link> </jats:named-content>’ with other related uncultivated clone sequences, we propose <jats:italic>Ardenticatenia</jats:italic> classis nov. and the subordinate taxa <jats:italic>Ardenticatenales</jats:italic> ord. nov. and <jats:italic>Ardenticatenaceae</jats:italic> fam. nov.</jats:p>