Synergistic mixtures of biocides and their use to control the growth of microorganisms in aqueous systems are disclosed. The method of using the synergistic mixtures entails adding an effective amount of a nitrogenous compound activated by an oxidant and at least one non-oxidizing biocide to an aqueous system. The amount of activated nitrogenous compound and non-oxidizing biocide is selected to result in a synergistic biocidal effect.