1. Potential differences were measured across the chorion of single eggs of Fundulus heteroclitus. The chorion was shown by three lines of evidence to be more permeable for cations than for anions: a. Concentration potentials were of such a sign that the dilute solution was positive to the concentrated. b. Concentration potentials with K salts of divalent and univalent anions were equal, whereas concentration potentials with chlorides of divalent cations were about half of those with K. c. Equal concentrations of different anions were eqipotential against the egg, whereas equal concentrations of different cations gave various potential differences whose magnitudes were in the same order inverted as the mobilities of those cations in free diffusion. 2. The difference between the permeability of the chorion for anions and its permeability for cations increased with dilution of the solution in which the egg was immersed. 3. In KCl and CaCl2 solutions the ratio of chorion permeability for anions to permeability for cations increased with the H ion concentration, and was inverted with sufficiently increased acidity. 4. The pH of the reversal point, where permeability for anions was equal to permeability for cations, depended on the salt used. For KCl in the concentrations used it lay in the neighborhood of 3.7.