The prokaryotic voltage-gated Na(+) channel, NaChBac, is one of a growing channel superfamily of unknown function. Here we show that Na(V)BP, the NaChBac homologue encoded by ncbA in alkaliphilic Bacillus pseudofirmus OF4, is a voltage-gated Na(+) channel potentiated by alkaline pH. Na(V)BP has roles in motility, chemotaxis, and pH homeostasis at high pH. Reduced motility of bacteria lacking functional Na(V)BP was reversed by restoration of the native channel but not by a mutant Na(V)BP engineered to be Ca(2+)-selective. Motile ncbA mutant cells and wild-type cells treated with a channel inhibitor exhibited behavior opposite to the wild type in response to chemoeffectors. Mutants lacking functional Na(V)BP were also defective in pH homeostasis in response to a sudden alkaline shift in external pH under conditions in which cytoplasmic [Na(+)] is limiting for this crucial process. The defect was exacerbated by mutation of motPS, the motility channel genes. We hypothesize that activation of Na(V)BP at high pH supports diverse physiological processes by a combination of direct and indirect effects on the Na(+) cycle and the chemotaxis system.