The relationship between movement of ions and the electrical properties of microorganisms (algae, fungi, and bacteria) are reviewed, with particular emphasis on the giant alga, Nitella, and the fungus, Neurospora. The hypothesis is presented that there are two basically different components to the membrane potential of both organisms: (1) one arising from the diffusion of sodium, potassium, and hydrogen ions down their chemical gradients, and (2) one associated with the utilization of energy and the active efflux of hydrogen ions, and attributed to an “electrogenic H+ pump.” Numerous discrepancies between the measured electrical properties of the algae or fungi and the predictions of ordinary ion-diffusion theory can be accounted for by such an H+ pump, and its existence is further supported by a few indirect experiments on the bacteria.