The2H nuclear magnetic resonance spectrum of mollisin from cultures of the fungusMollisiacaesiasupplemented with [2-2H3]acetate was consistent with biosynthesis of the metabolite via a single-chain polyketide. The presumption that chain scission accompanies dichlorination of a reactive methylene group in such a polyketide was supported by the detection of chloroperoxidase activity with monochlorodimedone as a substrate, and failure to detect chlorinating activity when the substrate was acetoacetyl coenzyme A. However, a decisive choice between single-chain and two-chain polyketide intermediates cannot be made with information provided by the techniques used so far.Keywords: biosynthesis, mollisin, deuterium labelling and NMR; deuterium labelling and NMR, mollisin biosynthesis; labeling of mollisin by deuterium, NMR analysis; mollisin biosynthesis, deuterium, labeling and NMR; NMR deuterium, mollisin biosynthesis.