Cylindrocladium quinqueseptatum Boedijn & Reitsma causes seedling blight and extensive defoliation on a wide variety of plants and is widely distributed in the humid tropics. It is a serious pathogen of Eugenia caryophyllata in Sri Lanka and has been identified as a potential pathogen on Hevea brasiliensis in the Eastern Hemisphere. All clove isolates: IMI 342173 (Rt), 359378 (Aw), 359379 (Rw) and 359380 (Kp) of C.quinqueseptatum tested were found to be capable of secreting toxic substances to the growing medium. Toxic culture filtrates proved to be thermostable (up to 100 oC) and hhost specific. The type and the size of the lesions produced by the crude toxin of different isolates varied markedly. Isolate, Rt produced the largest lesions (highest score) indicating its aggressive nature in toxin production while Aw and Rw isolates showed a mild reaction. When the reaction of Hevea clones towards the crude toxin was considered three main clusters of clones were distinguished through cluster analysis indicating the marked variation of Hevea clones grown in the Eastern Hemisphere in their sensityvity to the crude toxin. The Possibility of making use of this toxin as a tool in screening Hevea populations for Cylindrocladium leaf spot disease has also been discusseed.