A method for increasing the effectiveness of the microbicide propiconazole, (RS)-1-2-[(2,4-dichlorophenyl)-2-propyl-1,3-dioxalan-2ylmethyl]-1H-1,2,4-triazole, is described. In the method, propiconazole and a potentiator, an N-alkyl heterocyclic compound, its salt, or a mixture thereof, are applied to a substrate or aqueous system subject to the growth of microorganisms. The N-alkyl heterocyclic compound, its salt, or a mixture thereof is applied in an amount effective to increase the microbicidal activity of the microbicide. The N-alkyl heterocyclic compound has formula (I). The variable 'n' ranges from 5 to 17, and the heterocyclic ring defined by (II) is a substituted or unsubstituted ring having four to eight members. Microbicidal compositions are described where propiconazole and an N-alkyl heterocyclic compound, its salt, or a mixture thereof are present in a combined amount effective to control the growth of at least one microorganism. Methods for controlling the growth of microorganisms on various substrates and in various aqueous systems are also described. The combination of propiconazole and N-alkyl heterocyclic compound, its salt, or a mixture thereof is particularly useful as microbicidal in the leather industry, the lumber industry, the papermaking industry, the textile industry, the agricultural industry, and the coating industry, as well as in industrial process waters.