Allelopathic activity was investigated on the exudate of Mexican sunflower (Tithonia diversifolia (Hemsl) A. Gray) (Figure 2 and 3). The ethyl acetate soluble fraction of exudates from seeds inhibited a fungal germination. Sundiversifolide was identified in the active fraction by LC-ESI+/MS analysis (Figure 4). These results indicate that the sundiversifolide may have been related to the allelopathic activity as allelochemical in Mexican sunflower seeds (fruits).