<jats:p>A moderately thermophilic chemoheterotrophic bacterium, strain Mat9-16<jats:sup>T</jats:sup>, was isolated from microbial mats developed in hot spring water streams from Yumata, Nagano, Japan. Cells of strain Mat9-16<jats:sup>T</jats:sup>were strictly anaerobic, Gram-stain-negative, non-sporulating, non-motile and short to long rods (2.0–15.5 μm in length). Strain Mat9-16<jats:sup>T</jats:sup>grew fermentatively with optimum growth at 45 °C, pH 7.0–7.5 and 1 % NaCl (w/v). Phylogenetic analysis based on the 16S rRNA gene revealed that strain Mat9-16<jats:sup>T</jats:sup>was affiliated with an uncultivated lineage, and the nearest cultivated neighbours were green sulfur bacteria belonging to the class<jats:italic>Chlorobea</jats:italic>with 77–83 % sequence similarity. However, strain Mat9-16<jats:sup>T</jats:sup>could not grow phototrophically and did not possess light-harvesting structures, morphologically and genetically, such as the chlorosomes of green sulfur bacteria. On the basis of phenotypic features and phylogenetic position, a novel genus and species are proposed for strain Mat9-16<jats:sup>T</jats:sup>, to be named<jats:italic>Ignavibacterium album</jats:italic>gen. nov., sp. nov. (=NBRC 101810<jats:sup>T</jats:sup>=DSM 19864<jats:sup>T</jats:sup>). We also propose to place the cultivated bacterial lineage accommodating the sole representative Mat9-16<jats:sup>T</jats:sup>in a novel class,<jats:italic>Ignavibacteria</jats:italic>classis nov. In addition, we present a formal description of the phylum-level taxon<jats:italic>‘Chlorobi’</jats:italic>as<jats:italic>Chlorobi</jats:italic>phyl. nov.</jats:p>