A possibility of dissimilatory MnO2 reduction at extremely high salt and pH was studied in sediments from hypersaline alkaline lakes in Kulunda Steppe (Altai, Russia). Experiments with anaerobic sediment slurries demonstrated a relatively rapid reduction of colloidal MnO2 in the presence of acetate and formate as electron donor at in situ conditions (i.e., pH 10 and a salt content from 0.6 to 4 M total Na+). All reduced Mn at these conditions remained in the solid phase. A single, stable enrichment culture was obtained from the slurries consistently reducing MnO2 at pH 10 and 0.6 M total Na+ with formate. A pure culture of a haloalkaliphilic Mn-reducing bacterium obtained from the positive enrichment was phylogenetically closely related to the anaerobic haloalkaliphilic Bacillus arseniciselenatis isolated from Mono Lake (CA, USA). Bacillus sp. strain AMnr1 was obligately anaerobic, able to grow either by glucose fermentation, or respiring few nonfermentable substrates by using MnO2 as the electron acceptor. Optimal growth by dissimilatory MnO2 reduction was achieved with glycerol as electron donor at pH 9.5–10 and salt content between 0.4 and 0.8 M total Na+.