Gliotoxin is a redox-active nonribosomal peptide produced by Aspergillus fumigatus. Like many other disulfide-containing epipolythiodioxopiperazines, a bis-thiomethylated form is also produced. In the case of gliotoxin, bisdethiobis(methylthio) gliotoxin (BmGT) is formed for unknown reasons by a cryptic enzyme. Here, we identify the S-adenosylmethionine-dependent gliotoxin bis-thiomethyltransferase (GtmA), which converts dithiogliotoxin to BmGT. This activity, which is induced by exogenous gliotoxin, is only detectable in protein lysates of A. fumigatus deficient in the gliotoxin oxidoreductase, gliT. Thus, GtmA is capable of substrate bis-thiomethylation. Deletion of gtmA completely abrogates BmGT formation and we now propose that the purpose of BmGT formation is primarily to attenuate gliotoxin biosynthesis. Phylogenetic analysis reveals 124 GtmA homologs within the Ascomycota phylum. GtmA is encoded outside the gliotoxin biosynthetic cluster and primarily serves to negatively regulate gliotoxin biosynthesis. This mechanism of postbio-synthetic regulation of nonribosomal peptide synthesis appears to be quite unusual.