Provided are biocidal compositions comprising 2,2-dibromo-3- nitrilopropionamide and a compound selected from the group consisting of l-(3- chloroallyl)-3,5,7-triaza-l-azoniaadamantane tris(hydroxymethyl)-nitromethane and a hexahydrotriazine compound. The compositions are useful for controlling microorganisms in aqueous or water containing systems.