Abstract: A new method is disclosed for treating a concrete surface to be used in cact with sea water to prevent fouling of the surface, which comprises the steps of: (a) fluating the concrete surface with an aqueous solution of a compound selected from the group consisting of fluorosilicic acid, calcium fluorosilicate, magnesium fluorosilicate, and zinc fluorosilicate of sufficient concentration to strengthen the concrete surface and to prevent biodeterioration thereof; (b) treating the concrete surface with a biocide insoluble in water and selected from the group consisting of copper naphthenate, beta-naphthol, pentachlorophenol and tributyl-tin oxide to prevent biodeterioration of the concrete surface; and (c) fixing the concrete surface treated during steps (a) and (b) by treating the surface with the biocide defined in step (b) in solution with a polyesteric resin or an epoxy resin.