Degradative studies of fusicoccin, a phytotoxic glycoside produced by Fusicoccum amygdali resulted in a revised molecular formaula C36H56O12 and the characterisation of all the oxygen substituents. Thus fusicoccin was shown to be a glycoside with one methoxy, one acetoxy, and two hydroxy aglycone substituents, the sugar moiety consisting of an α-glucoside carrying one acetyl group at C-3 and a 1,1-dimethylallyl ether at C-6. The efficient incorporation of DL-[2-14C]mevalonic acid lactone into fusicoccin indicated a polyisoprenoid structure.