The cells of Halicystis impaled on capillaries reach a steady P.D. of 60 to 80 millivolts across the protoplasm from sap to sea water. The outer surface of the protoplasm is positive in the electrometer to the inner surface. The P.D. is reduced by contact with sap and balanced NaCl-CaCl2 mixtures; it is abolished completely in solutions of NaCl, CaCl2, KCl, MgSO4, and MgCl2. There is prompt recovery of P.D. in sea water after these exposures.