The formulas proposed earlier for gliotoxin and dethiogliotoxin (I, II) are not compatible with several recent experimental observations. These include the transformation of gliotoxin on short contact with alkaline alumina at 20° into the compound C13H10N2O2 (III), and the lack of satisfactory correspondence in chemical and spectroscopic properties between the synthetic 10-hydroxypyrazinoindole derivative IV and a degradation product, anhydrodethiogliotoxin, which had been assigned this chemical constitution. We have now deduced the structure V for gliotoxin. This expression permits the unambiguous interpretation of the extensive transformations of gliotoxin and dethiogliotoxin (VI) into degradation products of known structure, and leads directly to satisfactory formulations for several derived substances of hitherto unknown or uncertain structure and genesis. For example, the long-known anhydrodethiogliotoxin is now given the structure VII, and a new series of transformations, described below, is uniquely formulable in terms of the new structures.