The head piece separated from the A-ATP synthase of Halobacterium halobium hydrolyses ATP. This A1-ATPase is inhibited by nitrate but not by other chaotropic anions. The nitrate inhibition is noncompetitive with respect to ATP, reversible, and partially protected by chloride. In contrast, ATP synthase in situ (A1Ao-ATPase) is not inhibited by nitrate but apparently is inhibited by stronger chaotropic reagents, such as thiocyanate and trichloroacetate, which make the vesicle membrane permeable to protons. The mode of action of nitrate and chaotropic anions seems to differentiate A-ATPases from V-ATPases. Other strains of Halobacterium, Haloferax, Haloarcula, Halococcus and Natronobacterium, contain at least two polypeptides immunochemically similar to the two major subunits, (&agr;) (86x10(3 )Mr on SDS-PAGE) and &bgr; (64x10(3 )Mr), of the A-ATPase of Halobacterium halobium. When solubilized, membrane vesicles of these halobacteria hydrolyse ATP. Their ATPases are commonly sensitive to nitrate. They require high concentrations of the supporting salt but depend differently on chloride or sulfate/sulfite. The A-ATPases of Halobacteriaceae appear to diverge with respect to salt preference.