The concentrations of K+, Na+, and Cl− in the cytoplasm and the vacuole of Acetabularia mediterranea have been measured, as have the vacuolar concentrations of SO4−− and oxalate. The electrical potential difference between external solution, and vacuole and cytoplasm has been measured. The results indicate that Cl− and SO4−− are probably transported actively into the cell, and that active transport of Na+ is outwards. The results for K+ are equivocal. The fluxes of K+, Na+, Cl−, and S04−− into the cell and the effluxes of Na+ and Cl− have been determined. The Cl− fluxes are extremely large. In all cases the plasmalemma is the rate-limiting membrane for ion movement. A technique is described for the preparation of large, completely viable cell fragments containing only cytoplasm, with no vacuole.