The primitive marine pulmonateAmphibola crenataexhibits isosmotic intracellular regulation over the range of 25%‐150% sea water with the exception of 75%‐85% sea water where the cells swell isotonically by entry of water and inorganic solutes. Under hypo‐osmotic conditions, potassium is lost throughout the experimental salinity range, chloride is lost belowca50% seawater at the same rate as potassium, but the lowering of the sodium concentration is attributable solely to the increased hydration of the cells. Under hyperosmotic conditions, all three ions are accumulated, sodium and chloride to a greater extent than potassium. In all salinities, potassium is at a higher concentration, and sodium and chloride at lower concentrations, than in the extracellular fluids. Sodium is associated with chloride in the cells, and potassium with organic anions. There is a negative correlation between sodium and potassium concentrations. The distribution of potassium and chloride between the inside and outside of the cells does not accord with a Donnan equilibrium. It is concluded that the mechanisms of hypo‐osmotic cell volume regulation inAmphibolainitially involve reductions in intracellular potassium and amino acids, with chloride largely replacing amino acids as the osmotic effector in very low external salinities. Hyperosmotic regulation involves an increase in all three ions, particularly sodium.