A novel thermophilic anaerobic and microaerophilic bacterium (optimal growth in the presence of 5-10% O(2)), strain Nad S1(T) was isolated from the terrestrial hot spring of Hammam Sidi Jdidi, Nabeul, Tunisia. Cells were motile rods having a Gram-positive cell wall structure. Strain Nad S1(T) grew optimally at 55 degrees C (range 37-70 degrees C). Optimum pH for growth was 6.5-7.0. It was halotolerant growing with NaCl up to 7% (optimum concentration 1.5-3.0%). It grew chemoorganotrophically on various carbohydrates, organic-acids and amino-acids as energy sources, or chemolithotrophically on H(2) using nitrate, as terminal electron acceptor. Beside oxygen (under microaerobic conditions) and nitrate, nitrite was also used. Nitrate was completely reduced to N(2). No fermentation occurred. The genomic DNA G + C content was 41.8 mol%. Based on 16S rRNA gene sequence analysis, strain Nad S1(T) belongs to the Bacillaceae family within the class 'Bacilli'. Because of its phylogenetic and phenotypic characteristics, we propose this isolate to be assigned as a novel genus and a novel species within the domain Bacteria, Microaerobacter geothermalis gen. nov., sp. nov. The type strain is Nad S1(T) (=DSM 22679(T) =JCM 16213(T)).