Codium decorticatum maintains its internal osmotic pressure 2.3 bar above the osmotic pressure of the external medium, which results in a hydrostatic pressure difference (turgor) of 2.3 bar. During hypoosmotic stress the primary mechanism for decreasing the internal osmotic pressure is a decrease in active Cl− influx. The decreased Cl− influx is accompanied by a 6.9 mV depolarization of the negative vacuole potential, while there is no consistent change in the protoplast resistance. The net Cl− efflux which results is balanced primarily by a passive net loss of K+. During hyperosmotic stress the Cl− influx increases, accompanied by a slight (2.6 mV) hyperpolarization and a decrease of 90 Ω cm2 in the membrane resistance. A passive net uptake of Na+ accompanies the net active influx of Cl−. Following the restoration of normal turgor, which requires about 2 hr, concentrations of Na+ and K+ are slowly readjusted over a period of about 30 hr until the normal K+/Na+ ratio of 1.0 is restored.Codium regulates its turgor as rapidly in the dark as in the light, which suggests that the Cl− pump is not dependent directly on photosynthesis for its energy supply.