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The allelopathic potential of Ageratum conyzoides was investigated under different environmental stress conditions, including nutrient deficiency, physical damage, 2,4-D treatment, competition with Bidens pilosa, infection with Erysiphe cichoracearum, and feeding by Aphiids gossypii. The inhibitory effects of A. conyzoides volatiles on peanut (Arachis hypogaea), redroot amaranth (Amaranthus retroflexus), cucumber (Cucumis sativus), and ryegrass (Lolium multiforum) increased when plants were grown under nutrient-deficient conditions or in competition with B. pilosa; however, there was no difference with physical damage or 2,4-D treatment. Phytoinhibitory effects decreased under fungal infection and aphid feeding. Volatiles from A. conyzoides plants infected with E. cichoracearum or exposed to A. gossypii feeding inhibited or killed fungi and insects. Precocenes and their derivatives, monoterpenes, and sesquiterpenes were the major volatile components of A. conyzoides.