The first naturally occurring, sulphur-containing dioxopiperazine unsubstituted on nitrogen, cis-3,6-dibenzyl-3,6-bis(methylthio)piperazine-2,5-dione (Ib), was isolated from Aspergillus terreus via preparative thin-layer chromatography following feeding experiments with radio-labelled precursors (e.g., [¹⁴C]phenylalanine, [¹⁴C]-cyclo-(L-Phe-L-Phe), sodium [³⁵S]sulphate). Its gross structure was deduced from the ¹H NMR spectrum (cis-configuration demanded by optical activity) and confirmed by synthesis of the corresponding racemate. As the first natural dioxopiperazine having sulphur attached to the 3- and 6-positions without other changes, it shows that biosynthetic introduction of sulphur can occur immediately following cyclodipeptide construction in A. terreus. The corresponding epidisulphide (2d) was tested as a biosynthetic precursor of bisdethiobis(methylthio)acetylaranotin (BDA, 3) using doubly labelled (³H and ³⁵S) (2d). Incorporation of (2d) into (3) with low dilution suggests a precursor-product relationship, but the change in isotope ratio requires caution. Further studies (e.g., using separate enantiomers or related dithiols) are needed to clarify (2d)'s status as an intermediate in BDA biosynthesis.