In the course of thermoinactivation of the smaller izoenzyme and of a truncated form of glucoamylase from Aspergillus niger, we observed the formation of disulfide-linked dimers. Molecular modelling studies indicated and the cyanogen bromide cleavage pattern gave corroborating evidence that it is the Cys 222-449 intramolecular disulfide bond that opens up and reforms intermolecularly to yield dimers. Since the dimer displayed the same activity as the native enzyme toward maltopentaose and soluble starch and showed increased thermostability at temperatures above 68 degrees C, it appears a prospective candidate for industrial usage.