Bacillus subtilis exhibited an inducible K+-transporting ATPase activity with apparent Km and maximum velocity Vmax of 12.9 microM and 25.1 micromol x min(-1) x (g cell protein)(-1), respectively, when cultivated on a synthetic medium containing less than 400 microM K+. Due to this enzyme, the growth rate of the bacterium in synthetic medium was not changed down to 115 microM K+, and the bacterium was able to grow down to 20 microM K+. The limiting K+ concentration was higher in media with osmolarity increased by NaCl or sucrose. The ATPase was inhibited by micromolar concentrations of vanadate (Ki = 1.6 microM). The ATPase activity was not stimulated by any other monovalent cation. The subunit of this ATPase, with an Mr of 52000, covalently bound the gamma phosphate group of ATP. This phosphorylated intermediate was unstable in neutral and basic pH as well as in the presence of potassium and was stable in acid pH. The enzyme did not show immunological cross-reactivity with antibody against Kdp ATPase of Escherichia coli.