A novel, irregular, coccoid-shaped archaeum was isolated from a hydrothermally heated black smoker wall at the TAG site at the Mid Atlantic Ridge (depth 3650 meters). It grew at between 90 degrees C and 113 degrees C (optimum 106 degrees C) and pH 4.0-6.5 (optimum 5.5) and 1%-4% salt (optimum 1.7%). The organism was a facultatively aerobic obligate chemolithoautotroph gaining energy by H2-oxidation. Nitrate, S2O3(2-), and low concentrations of O2 (up to 0.3% v/v) served as electron acceptors, yielding NH4+, H2S, and H2O as end products, respectively. Growth was inhibited by acetate, pyruvate, glucose, starch, or sulfur. The new isolate was able to form colonies on plates (at 102 degrees C) and to grow at a pressure of 25000 kPa (250 bar). Exponentially growing cultures survived a one-hour autoclaving at 121 degrees C. The GC content was 53 mol%. The core lipids consisted of glycerol-dialkyl glycerol tetraethers and traces of 2,3-di-O-phytanyl-sn-glycerol. The cell wall was composed of a surface layer of tetrameric protein complexes arranged on a p4-lattice (center-to-center distance 18.5 nm). By its 16S rRNA sequence, the new isolate belonged to the Pyrodictiaceae. Based on its GC-content, DNA homology, S-layer composition, and metabolism, we describe here a new genus, which we name Pyrolobus (the "fire lobe"). The type species is Pyrolobus fumarii (type strain 1A; DSM 11204).