The acidophilic bacteria comprise an environmentally important group that includes pathogens. A fundamental requirement for existence in strongly acidic environments is generation of a large pH gradient (delta pH) to maintain the cytoplasmic pH near neutrality and safeguard the acid-labile cell constituents. These organisms require the capacity to extrude H+ across the thermodynamic barrier imposed by the delta pH. They also need special transport proteins that can function in strongly acidic environments. The quintessential device by which the acidophiles fulfil these requirements by generating a positive-inside membrane potential (delta psi) that mitigates both the force against which H+ must be extruded, as well as the influxing force of protons into the cells. The delta psi is generated through passive as well as active mechanisms. The former constitutes an H+ diffusion as well as a Donnan potential, and the latter involves electrogenic circulation of Cl-, and the operation of an electrogenic H+/K+ (Na+) antiporter.