The water permeability of hardened coral concrete was investigated to learn the effects of mix design factors. The independent variables were aggregate type and source, watercement ratio, cement content, and pozzolanic admixture. The test data demonstrated that the least permeable coral concrete contained Eniwetok aggregate and after 3 months exposure had a coefficient of permeability approximating 0.000001 inches per hour for the liquid phase and approximating 0.000005 pounds per square inch per hour for the vapor phase. These values were equivalent, for all practical purposes, to the corresponding coefficients of the least permeable reference concrete. The least permeable coral concrete had a gross water-cement ratio not exceeding 5.4 gallons of water per bag of cement and a cement factor of 9.0 bags per cubic yard of concrete. The use of calcined opaline shale, as a partial replacement of portland cement, served to make the coral concrete test specimens stronger, more watertight, and more damp-proof than was otherwise possible, all other factors being equal. (Author).