Concentrations of the major component ions (K, Na and Cl) of sea water and cells of Enteromorpha intestinalis (L.) Link, were determined. Measurements of the electric potential difference across the plasmalemma and the influx and efflux of each ion indicate that K and Cl are actively imported and Na actively exported. Estimates of membrane permeability to the passive diffusive fluxes of these three ions alone have been used to explain the origin of the cell membrane potential. Tracer ion exchange kinetics are taken as showing that the tonoplast is more permeable than the plasmalemma to each ion.