Abstract: The present invention relates to a method for protecting surfaces (S) which are in contact or come into contact with a water-containing medium (M) against biological macro-fouling, wherein 1) S is electrically conducting and 2) such a potential (P) fluctuating over time is applied to S that it inhibits the growth of organisms that live in M and/or propagate therein and which have the tendency to form deposits on S, characterised in that P does not assume values that are higher than the corrosion potential of S in M and the average value of P is lower than the said corrosion potential.