An ATPase whose activity was stimulated by K+ was identified in Rhizobium sp. UMKL 20. The synthesis of the ATPase was repressed by high levels of K+. The enzyme had a pH optimum of about 8.0. It was highly specific for cations and only K+ appeared to be able to stimulate the enzyme. In terms of divalent cation specificity, both Mn2+ and Mg2+ stimulated K+-ATPase activity. ATP was the only nucleotide capable of supporting substantial activity. Vanadate was an inhibitor of the enzyme.