Novel halophilic, alkalithermophilic, Gram-type-positive bacterial strains were isolated from sediment of alkaline, hypersaline lakes of the Wadi An Natrun, Egypt. Cells of strain JW/NM-WN-LFT were rod-shaped, non-spore-forming and non-motile. Strain JW/NM-WN-LFT grew (at pH55 degrees C 9.5) between 35 and 56 degrees C, with an optimum at 53 degrees C. The pH55 degrees C range for growth was 8.3-10.6, with an optimum at pH55 degrees C 9.5 and no growth at pH55 degrees C 8.2 or below, or at pH55 degrees C 10.8 or above. At the optimum pH and temperature, the strain grew in the Na+ range of 3.1-4.9 M (1.5-3.3 M added NaCl) and optimally between 3.3 and 3.9 M Na+ (1.7-2.3 M added NaCl). Strain JW/NM-WN-LFT utilized fructose, cellobiose, ribose, trehalose, trimethylamine, pyruvate, Casamino acids, acetate, xylose and peptone as carbon and energy sources. Fumarate (20 mM), S2O3(2-) (20 mM), NO3- (20 mM) and iron(III) citrate (20 mM) were utilized as electron acceptors. During growth on sucrose, the isolate produced acetate and formate as major fermentation products. Main cellular fatty acids were iso-branched 15:0, i17:0 dimethylacetal and 16:0 dimethylacetal. The G+C content of genomic DNA was 40.4 mol% (HPLC). On the basis of genotypic and phenotypic characteristics, it is proposed that strain JW/NM-WN-LFT represents a novel genus and species, Natranaerobius thermophilus gen. nov., sp. nov. The type strain is JW/NM-WN-LFT (=DSM 18059T=ATCC BAA-1301T). Based on 16S rRNA gene sequence analysis, the strain forms a novel lineage within the class 'Clostridia' and clusters with uncultivated bacteria and unidentified strains retrieved from alkaline, hypersaline environments. The phylogenetic data suggest that the lineage represents a novel family, Natranaerobiaceae fam. nov., and order, Natranaerobiales ord. nov.