Histological study confirmed rapid and direct penetration of Corynespora cassiicola (12 h after inoculation) throughout the lower epidermis, observed also in previous work. The fungal colonisation of leaf tissues was intracellular, irrespective of clonal resistance. For the susceptible clone, fungus penetration led to an important collapse of abaxial epidermis and to cell disorganisation in front of the hyphal progression, characterised by nuclear degeneration and absence of starchy grains. This observation demonstrated that C. cassiicola behaved as a necrotrophic fungus. For the resistant clone, C. cassiicola invasion was restricted to a few cells leading to a hypersensitive- like reaction. The production of toxin in culture filtrate of the fungus, was confirmed. This toxin, named cassiicolin, reproduced similar leaf disease symptoms as observed after fungal inoculation. The role of the host-selective toxin in C. cassiicola pathogenesis was demonstrated using toxin antibodies and pure toxin. Results have shown that cassiicolin was essential for pathogenicity and can be considered as the primary determinant to C. cassiicola pathogenesis. A close correlation between sensitivity of Hevea clone to cassiicolin and their susceptibility to the fungus has been proven using an aggressive isolate from Philippines. Higher toxin concentrations could overcome the resistance of GT 1 which exhibited resistance to lower concentrations. Thus, clonal resistance to cassiicolin was not absolute and seemed to be strictly linked to the toxin concentration tested. The amount of toxin production by 11 C. cassiicola isolates from different countries was positively correlated to their pathogenicity. On the basis of these results which must be confirmed with a wide range of isolates, the using of cassiicolin for assessing the level of resistance of Hevea clones to C. cassiicola is discussed.