Weeds are considered undesirable plant species that negatively interferes in human interests, releasing into the environment secondary metabolites that may affect the growth and development of other biological systems, which is known as allelopathy.1 The Eragrostis plana Nees (Poaceae) are invasive plants and it is believed that the allelopathic interactions have guaranteed the dominance of this species on the other plants, in addition, it is difficult to control by commercial herbicides,2 which arouses the interest for its phytochemical study aiming to achieve a generation of a new bio-herbicide. The goal of this project was to evaluate the allelopathic potential of root extracts of E. plana Nees, besides isolating and identifying the secondary metabolites of the extract with better allelopathic activity. Seeds of Ipomoea grandifolia and Euphorbia heterophylla were used as recipient species. The concentrations of aqueous solutions of crude extracts (petroleum ether, ethyl acetate and methanol) used in the germination bioassays were 100, 200, 300, 400 and 500 ppm, and all bioassays (triplicate) used distilled water and solvent as control. The germination was monitored for 7 day period, and on the last day, the growth of the rootlet and the shoot of the seedlings of both recipients species were measured. The data were processed and analyzed using a generalized linear model to define which variables (G% (percentage of germination), TMG (mean germination time), GSI (germination speed index), seedling shoot and rootlet) were affected by the tested extracts. In sequence, the extract that presented best allelopathic activity was fractionated by column chromatography and the fractions were analyzed by liquid chromatography coupled to mass spectrometry (LC/MS). The allelopathic tests showed that the petroleum ether extract increased the TMG and decreased the rootlet growth, causing a negative effect in the seeds of both receptor species. Based on these results, it was decided to purify the petroleum ether extract from the roots by column chromatography, which resulted in 155 fractions that were grouped in 14 new ones. The LC/MS analysis of fraction F-19 showed the presence of seven substances, with molecular ions and fragmentation characteristic of diterpenes, pimarane type. A peak in 15 minutes presented quasi-molecular ion [M+H]+=303 m/z and the loss of two water molecules [M+H-H20]+=285 m/z and [M+H-2H20]+=267 m/z. Its fragmentation was proposed based on Pinto et al. (2016). Two other substances presented quasi-molecular ion [M+H]+=301 m/z, indicating an additional unsaturation with respect to the first substance and [M+H]+=315 m/z, probable presence of two carbonyl groups. Diterpenes are found in the genus Eragrostis which some have been showing allelopathic activity4.