Provided are biocidal compositions comprising 2,2-dibromo-3-nitrilopropionamide and a compound selected from the group consisting of 1-(3-chloroallyl)-3,5,7-triaza-1-azoniaadamantane; tris(hydroxymethyl)-nitromethane; and a hexahydrotriazine compound. The compositions are useful for controlling microorganisms in aqueous or water containing systems.