Analysis of the 13C and 15N n.m.r. spectra of [15N6]chetomin biosynthesized by Chaetomium cochliodes established that the sporidesmin-like and 3-(o-skatyl)-3,6-epidithiopiperazine-2,5-dione fragments are linked by a bond between the indole nitrogen and the quaternary P-indoline carbon. Chetomin, a toxic metabolite of Chaetomium spp, is thought to be associated with poor growth in young ruminants. Initial chemical and spectroscopic studies on the metabolite and its derivatives indicated the presence of a sporidesmin-like residue bonded to a 3,6-epidithiopiperazine-2,5-dione system as shown in (1). Structure (2) was selected because the 1H n.m.r. evidence showed that one N-methyl group (δ, 3.00) was chemically shifted from the other two which were, fortuitously, chemically equivalent (6H 3.16). In a recent preliminary account of our 13C and 15N n.m.r. studies of [15N]chetomin biosynthesised by Chaetomium cochliodes it was established that the metabolite had structure (3), and here the detailed evidence in support of this structure is presented.