A novel extremely thermophilic bacterium was isolated from the world's deepest sea-floor, the Mariana Trench Challenger Deep at a depth of 10,897 m. Cells were Gram-reaction variable, non-spore-forming and non-motile rods without flagella. Growth was observed between 50 and 80 degrees C (optimum: 74-76 degrees C; 90 min doubling time), pH 5.4 and 9.5 (optimum: pH 7.0-7.5) and 0.5 and 5% sea salts (optimum: 2% sea salts). The isolate was a strictly aerobic heterotroph capable of utilizing as sole energy and carbon source: yeast extract, peptone, cellulose, starch, chitin, casein, Casamino acids, a variety of sugars, carboxylic acids and amino acids. The G + C content of the genomic DNA was 72.5 mol%. Phylogenetic analysis based on 16S rRNA sequences placed this aerobic, high-G + C-content bacterium among the members of the Gram-positive, low-G + C-content anaerobic thermophilic bacteria within the Bacillus-Clostridium subphylum. On the basis of the physiological and molecular properties of the new isolate, the name Thermaerobacter marianensis gen. nov., sp. nov. (type strain 7p75aT = JCM 10246T) is proposed.