Atribacteria (OP9), candidate phylum with no representatives in pure culture, is found in various anaerobic environments worldwide. “Caldatribacterium”, a lineage within Atribacteria that is predicted to be a strictly anaerobic sugar fermenter based on cultivation-independent genomic analyses, is currently being maintained in lab enrichment cultures with fucose as its sole growth substrate. Metagenomics and 16S rRNA gene tag sequencing indicated that the fucose culture was a co-culture of “Caldatribacterium” and an uncultivated member of the genus Thermodesulfobacterium. Due to failed attempts to isolate “Caldatribacterium” by dilution-to-extinction and plating, it was hypothesized that “Caldatribacterium” is dependent in some way on the Thermodesulfobacterium. To better understand the possible interaction, multiple isolates of the sulfate reducer were obtained under sulfate-reducing conditions with H2 as an electron donor, and one of the isolates was characterized. Whole genome and 16S rRNA gene sequence comparisons of the isolate and other related members of the genus Thermodesulfobacterium suggested the isolate represents a distinct species in this genus, for which the name T. auxiliatoris is proposed. T. auxiliatoris was capable of using H2, formate, and lactate as sole electron donors, but not fucose or other sugars, suggesting that its growth in the co-culture might be dependent on one or more fermentation substrates produced by “Caldatribacterium”. Addition of T. auxiliatoris to highly diluted samples of the co-culture that likely contained only “Caldatribacterium”, which did not exhibit growth on their own, demonstrated that T. auxiliatoris was sufficient to support growth of “Caldatribacterium” on fucose. When this dilution experiment was repeated with various other organisms and substrates, it was found that several other thermophilic sulfate reducers (T. commune, T. hveragerdense, or Thermodesulfovibrio yellowstonii) could also support growth, as well as supernatant from the T. auxiliatoris pure culture or yeast extract. This last finding allowed for isolation of “Caldatribacterium”, which could form colonies on solid media when yeast extract and casamino acids were present. Fluorescent in situ hybridization and nanometer-scale secondary ion mass spectrometry demonstrated that “Caldatribacterium” took up a variety of sugars and amino acids in mixed culture, and that addition of acetate or bicarbonate, substrates of T. auxiliatoris, stimulated sugar uptake in “Caldatribacterium”. These results support a model where T. auxiliatoris and “Caldateribacterium” are dependent on each other in co-culture on fucose, where “Caldatribacterium” provides growth substrates for T. auxiliatoris, which in turn provides “Caldatribacterium” with some sort of soluble, essential compound(s) that can be produced by other sulfate reducers and are present in yeast extract. Further characterization of the “Caldatribacterium” isolate, the first representative of the phylum Atribacteria, will allow for detailed study of its metabolic capabilities that can be extended to other members of this phylum. Further analysis of responses of T. auxiliatoris and “Caldatribacteirum” when grow in co-culture and the specific metabolite(s) that are exchanged between the two organisms could allow for testing whether these interactions occur in more complex, natural systems.