The concentration of the preformed antifungal compound 1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15-diene in unharvested avocado fruit was almost doubled for a period of 3 days following inoculation by Colletotrichum gloeosporioides. The natural diene concentration in the peel and flesh of harvested fruits decreased between the first and third day after harvest but the initial level was regained faster the more mature the fruits were when harvested. A further decline in the concentration of the antifungal compound occurred during fruit ripening. If freshly harvested fruits were inoculated with C. gloeosporioides, the concentration of the diene was always enhanced 2???3 days later. Four days after inoculation of harvested fruits, the diene concentration in the peel and in the flesh of control fruit tissue was 490 and 650 ??g g-1 fresh weight, respectively, compared with 1900 and 3260 ??g g-1 fresh weight on the inoculated tissue. Mechanical wounding enhanced the concentration over a 24 h period only of the diene in the peel and the flesh. Fruit inoculation with the wound pathogens, Diplodia natalensis and Penicillium digitatum at harvest, did not increase the concentration of the diene. The diene concentration in freshly-harvested and peeled fruits was not enhanced by inoculation with C. gloeosporioides or treatment of the flesh with purified endopolygalacturonase of C. gloeosporioides. It is suggested that the concentration of the preformed diene may be induced by the interaction between C. gloeosporioides and the fruit peel.