The unfertilized egg of the sea urchin, Arbacia, is a living osmometer. The rate of swelling of these cells in hypotonic sea water, over wide ranges of temperature and concentration, follows the unimolecular equation, which should hold for a diffusion process. The temperature coefficient of the rate of swelling is high (2-3), for a change in temperature of 10[degree], corresponding to a chemical rather than to a diffusion process. The Arrhenius" temperature characteristic, [mu], ranges from 12,000 to 19,000. The rate of swelling is greatly affected by the osmotic pressure of sea water in a way not explainable on the grounds of a pure diffusion process. It is concluded that unknown changes in the semipermeable membrane modify the effects of diffusion.