A new extremely haloalkaliphilic, chemoorganotrophic, homoacetogenic bacterium strain Z-7937(T)(T-type strain) was isolated from the bottom mud of the soda-depositing Lake Magadi, Kenya. It is an obligately anaerobic, motile, Gram-negative, spore-forming rod growing in the pH range pH 8.1 to 10.7 and optimally in the range pH 9.7 to 10.0 under conditions of high alkalinity caused by saturation with trona. It has an obligate requirement for sodium carbonate and chloride ions. The optimum salt concentration for growth is in the range 12-15% wt/vol, and growth occurs within the range from 10% to 26%. Strain 2-7937(T) is a mesophile with an optimal temperature for growth of 37 degrees C, and a maximum of 42 degrees C. The G + C content of strain Z-7937(T) is 31.9 mol%. A limited number of compounds are utilized, including lactate, ethanol, pyruvate, glutamate, and propanol. Acetate is the main end product. 16S rDNA sequence analysis shows strain Z-7937(T) to be a member of the order Haloanaerobiales and to represent a new branch within the family Halobacteroidaceae. On the basis of its novel physiology and phylogenetic position, we propose strain Z-7937 as a new species of a new genus, Natroniella acetigena gen. nov. sp. nov. The type strain is Z-7937(T) (= DSM 9952).