During studies of moderately halophilic strains of Ectothiorhodospira from steppe soda lakes, we found a novel group of bacteria related to Ectothiorhodospira haloalkaliphila with salt optima at 50-80 g NaCl l(-1). Phylogenetic analysis using 16S rRNA gene sequences of strains from soda lakes in Mongolia, Egypt and Siberia revealed separation of the group of new isolates from other Ectothiorhodospira species, including the closely related Ect. haloalkaliphila. DNA-DNA hybridization studies demonstrated that the new isolates form a homogeneous group at the species level, but at the same time are distinct from related species such as Ect. haloalkaliphila, Ect. vacuolata, Ect. shaposhnikovii and Ect. marina. The new isolates are considered to be strains of a novel species, for which the name Ectothiorhodospira variabilis sp. nov. is proposed, with the type strain WN22(T) (=VKM B-2479(T) =DSM 21381(T)). Photosynthetic pigments of the novel species are bacteriochlorophyll a and carotenoids of the spirilloxanthin series with spirilloxanthin and derivatives thereof, together with small amounts of lycopene and rhodopin. Gas vesicles are formed by most of the strains, particularly in media containing yeast extract (0.5 g l(-1)) and acetate (0.5-2.0 g l(-1)). Sequence analysis of nifH (nitrogenase) and cbbL (RuBisCO) confirmed the assignment of the strains to the genus Ectothiorhodospira and in particular the close relationship to Ect. haloalkaliphila. The novel species Ect. variabilis is found in soda lakes separated by great geographical distances and is an alkaliphilic and halophilic bacterium that tolerates salt concentrations up to 150-200 g NaCl l(-1).