The elucidation of the structure of gypsetin, a new inhibitor of acyl-CoA: cholesterol acyltransferase, is described in this paper. By spectroscopic and X-ray crystallographic analyses, the structure of gypsetin has been determined to be 8a, 16a-dihydroxy-5a,l3a-bis[1,1-dimethylallyl]-[1]benzazolidine[3''',2''':4'',5'']azolidino[1'',2'':4',5'][1,4]perhydrodiazin[1',2':1,5]azolidino[2,3-b]-[1]benzazolidine-7,15-dione.