Procedures for calculating returns per acre‐foot of water were developed in western United States irrigation projects and have been applied to supplemental irrigation in the humid eastern areas. These procedures constitute an operational definition of a marginal product of water, but the definition rests on a different theoretical basis in the two cases and does not apply to intermediate conditions. No general notion of marginal revenue of water appears meaningful for manufacturing uses, which are largely nondepleting. Comparisons made between the value of water in farming and in industrial uses are of doubtful value, particularly if they are extended to the humid regions. A method is proposed here for estimating the ‘marginal value’ of water intake in manufacturing. The method is based on the cost of the internal economy of water in an industrial plant. It may be used to compare the relative productivity of water in depleting and nondepleting uses, or to compare the returns to industry with the costs of public development of a water supply. Some estimates of marginal value are derived from data supplied by the Bethlehem Steel Company plant at Sparrows Point, Mar