the amazon biodiversity may provide plants whose chemical substances are capable of controlling weeds. in this study we report the isolation and identification of five stilbenes from the leaves of "timbó vermelho" (deguelia rufescens var. urucu): 4-methoxylonchocarpene (1); 3,5-dimethoxy-4′-hydroxy-3′-prenyl-trans-stilbene (2), lonchocarpene (3), 3,5-dimethoxy-4′-o-prenyl-trans-stilbene (4) and pterostilbene (5). compounds 2 and 4 are new natural products although 2 has been previously cited as synthesis product. potential allelopathic activity for 1, 2 and 4 was evaluated over seed germination and plant growth of mimosa pudica weed. the observed effects on seed germination did not vary significantly (p > 0.05) when the analysis of phytotoxicity was performed with the substances alone, the maximum inhibition did not exceed 20%. the most intense inhibitions on radicle and hypocotyl development were found for compound 4 (p < 0.05). when tested in pairs, showed antagonism for seed germination and synergism for radicle and hypocotyl development.