ATPase was detected in the membranes of a motile Streptococcus. Maximal enzymic activity was observed at pH 8 and ATP/Mg2+ ratio of 2. Mn2+ and Ca2+ could replace Mg2+ to some extent. Besides ATP, GTP and ITP were substrates. The enzyme was inhibited by N,N'-dicyclohexylcarbodiimide but not by sodium azide, uncouplers or bathophenanthroline. An electrochemical gradient of protons, which was artificially imposed across the membranes of Streptococcus cells by manipulation of either the K+ diffusion potential or the transmembrane pH gradient, led to ATP synthesis. ATP synthesis was abolished by proton conductors, an inhibitor of the ATPase or an increase in the extracellular K+ concentration. A comparison between the phosphate potential and the electrochemical proton gradient showed that the data found are in agreement with a stoichiometry of 2 protons translocated per molecule ATP synthesized.