Novel strains of anaerobic, halophilic, alkalithermophilic bacteria were isolated from sediments of the alkaline, hypersaline lakes of the Wadi An Natrun, Egypt. Strains JW/NM-WN-LUT (and related strains) and JW/NM-WN-LH1T were non-spore-forming, non-motile bacteria with a Gram-type positive cell wall. Optimal growth of strain JW/NM-WN-LUT occurred at pH55 degrees C 9.5, 52 degrees C and with 3.7 M Na+ (2.2 M added NaCl). Optimal growth of strain JW/NM-WN-LH1T occurred at pH55 degrees C 9.9, 51 degrees C and with 3.9 M Na+ (2.3 M added NaCl). Both strains were obligately anaerobic and chemoorganotrophic; producing lactate and acetate as organic acids from pyruvate in different ratios. The DNA G+C contents of strains JW/NM-WN-LUT and JW/NM-WN-LH1T were 41.0 and 42.0 mol%, respectively. The main cellular fatty acids in both strains were anteiso-branched 15:0 and iso-branched 15:0. Phylogenetic analysis showed that both novel isolates belonged to the family Natranaerobiaceae within the order Natranaerobiales. Based on genotypic and phenotypic data, strain JW/NM-WN-LUT (=DSM 18760T=ATCC BAA-1443T) represents the type strain of a novel species, Natranaerobius trueperi sp. nov. Strain JW/NM-WN-LH1T represents a novel genus and species within the family Natranaerobiaceae, Natronovirga wadinatrunensis gen. nov., sp. nov. The type strain of the type species is JW/NM-WN-LH1T (=DSM 18770T=ATCC BAA-1444T).