Bioenergetic characteristics of Na⁺ pump-defective mutants of a marine bacterium Vibrio alginolyticus were compared with those of the wild type and revertant. Generation of membrane potential and motility at pH 8.5 in the mutants were completely inhibited by a proton conductor, carbonylcyanide m-chlorophenylhydrazone, whereas those in the wild type or revertant were resistant to the inhibitor. Motility and amino acid transport were driven by the electrochemical potential of Na⁺ not only in the wild type or revertant but also in the mutants. In the absence of the proton conductor, motility and amino acid transport of the mutants did not significantly differ from those of the wild type or revertant even at pH 8.5, where the Na⁺ pump has maximum activity. Therefore, the electrochemical potential of Na⁺ in the mutants seemed to be maintained at a normal level by a respirationdependent H⁺ pump and Na⁺/H⁺ antiporter. On the other hand, growth of the mutants became defective as the medium pH increased, especially on minimal medium. These results indicate that the Na⁺ pump is an important energy-generating mechanism when nutrients are limited at alkaline pH.