The Thermoanaerobacterales is a polyphyletic order of bacteria placed within the polyphyletic class Clostridia, and encompassing four families: the Thermoanaerobacteraceae, the Thermodesulfobiaceae, the Thermoanaerobacterales Family III.

Incertae Sedis, and the Thermoanaerobacterales Family IV.

[1] This order is noted for the species' abilities to survive in extreme environments without oxygen and of relatively elevated temperatures for a living being (up to 80-90 °C).

[2] The Thermoanaerobacterales, as previously mentioned, is polyphyletic, and consists of numerous morphologically similar clades: Carboxydothermus Syntrophaceticus Thermacetogenium Moorella Calderihabitans Koleobacter Zhaonella Desulfitibacter Desulfitispora Syntrophomonadaceae Caldanaerobius Caloribacterium Calorimonas Thermoanaerobacterium * [incl.

Thermohydrogenium] Dictyoglomus Caldicellulosiruptor Brockia Hydrogenibacillus Thermodesulfitimonas Desulfothermobacter Ammonifex Thermosediminibacterales * Desulfovirgula Thermanaeromonas Caldanaerobacter Thermoanaerobacter Thermacetogenium Syntrophaceticus Zhaonella Calderihabitans Desulfitibacter Thermanaeromonas Moorella Carboxydocella "Thermincolales" Carboxydothermus "Desulforudaceae" Ammonifex Thermodesulfitimonas "Desulfotomaculales" Koleobacterales Thermosediminibacterales * Caldicellulosiruptor Calorimonas Caldanaerobius Thermoanaerobacterium Caldanaerobacter Thermoanaerobacter "Clostridiia" s.s.