The marine bacterium, Vibrio alginolyticus, normally requires sodium for motility. We found that lithium will substitute for sodium. In neutral pH buffers, the membrane potential and swimming speed of glycolyzing bacteria reached maximal values as sodium or lithium concentration was increased. While the maximal potentials obtained in the two cations were comparable, the maximal swimming speed was substantially lower in lithium. Over a wide range of sodium concentration, the bacteria maintained an invariant sodium electrochemical potential as determined by membrane potential and intracellular sodium measurements. Over this range the increase of swimming speed took Michaelis-Menten form. Artificial energization of swimming motility required imposition of a voltage difference in concert with a sodium pulse. The cation selectivity and concentration dependence exhibited by the motile apparatus depended on the viscosity of the medium. In high-viscosity media, swimming speeds were relatively independent of either ion type or concentration. These facts parallel and extend observations of the swimming behavior of bacteria propelled by proton-powered flagella. In particular, they show that ion transfers limit unloaded motor speed in this bacterium and imply that the coupling between ion transfers and force generation must be fairly tight.The rotation of bacterial flagella (6) (25). The role of proton transfers in limiting motor speed has been probed by study of hydrogen isotope effects. Isotope effects were absent during tethered cell rotation (9, 18) but prominent during rapid rotation of flagellar bundles in free-swimming cells (9,25).An increasing number of bacterial species are being identified in which flagellar rotation depends on sodium gradients (4,11,12,16,17,33,34). Identification, upon comparison with the proton motors, of the changes in machinery and mechanism that provide the basis for the sodium dependence should yield valuable clues about the coupling between ion transfers and force generation. We began work on Vibrio alginolyticus guided by this consideration. We chose V. alginolyticus since it (i) has well-characterized physiology (35, 36), (ii) exhibits good motility at neutral pH, at which proton pumps maintain the membrane potential independently of sodium (36)