Abstract: 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 the formula: The variable “n” ranges from 5 to 17, and the heterocyclic ring defined by 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.