Abstract: The invention relates to a process for the biological purification of a water containing ammonium perchlorate, characterized in that aqueous ammonium perchlorate solution is placed in contact, in a first, aerobic reactor, with an activated sludge in the presence of at least one source of mineral carbon and nutritional elements for the metabolism of the microorganisms, and then in that the liquid effluent leaving the first reactor is placed in contact, in a second, anoxic reactor, with an activated sludge, in the presence of at least one source of organic carbon and nutritional elements for the metabolism of the microorganisms. The process makes it possible to treat aqueous solutions containing very large amounts of ammonium perchlorate and to convert both the ammonium ions into molecular nitrogen and the perchlorate ions into chloride ions.