Abstract: The invention provides a novel and efficient method for preventing biofouling on the surface of a solid body continuously in contact with bacteria-containing sea water, such as the heat transfer surface in a heat exchanger using sea water as the cooling medium, caused by the attachment of the proliferated bacteria. The method comprises adding bacteriophages capable of lysing the bacteria responsible for the biofouling of the surface to the sea water brought into contact with the surface so that the sea water can be efficiently and inexpensively sterilized prior to contacting with the surface not to cause biofouling absolutely without the problem of environmental pollution.