INTRODUCTION: Sphagneticola trilobata (L.) Pruski (Acmella brasiliensis, Wedelia paludosa) (Asteraceae) is a native species distributed throughout Brazil. There are several reports in the literature about the phytotoxic potential of the Asteraceae family, especially the genera Achillea and Solidago. Considering the phytotoxic activity described for this family previously and taking into account the importance of the discovery of natural alternatives for the control of weeds to the detriment of the exacerbated use of synthetic herbicides, our research interest was aroused by the phytotoxic potential of Sphagneticola trilobata, a recognized invasive species. The objectives of the work are to evaluate the phytotoxic activity of the aerial parts of S. trilobata, and to identify the secondary metabolites thereof. METHODOLOGY: The crude extract of the leaves of S. trilobata was obtained by maceration in 70% ethanol. Part of this extract was taken to the liquid-liquid partition in hexane, dichloromethane, ethyl acetate, obtaining different fractions and finally the aqueous residue. To evaluate the phytotoxic activity, bioassays were performed in Petri dishes containing test solution at a determined concentration (1000, 500 and 250 ppm) and ten seeds of Lactuca sativa (lettuce). Petri dishes containing 0.1% DMSO served as controls without treatment whereas menadione at 143 ppm (IC50) was used as a positive control. Seedling growth was evaluated by measuring the length of the hypocotyls and rootlets 5 days after seeds introduction. The determination of the total phenol content of the extract and fractions was performed by the spectrophotometric quantification method with the Folin-Ciocalteu reagent. RESULTS: In the bioassays, the L. sativa rootlets were more sensitive to the inhibitory effect of the phytotoxic substances of S. trilobata than the hypocotyls. In the highest concentration, the percentage of inhibition of the root growth was 40.66%, 85.49%, 98.55%, 69.36% and 79.32%, respectively, for the hydroalcoholic extract, hexane fraction, dichloromethane fraction, ethyl acetate fraction and the aqueous residue. For the ethyl acetate fraction, it was possible to determine the phytotoxic IC50, with a value of 376 ppm. The TLC analysis enabled to identify the presence of flavonoids in the ethyl acetate fraction by the appearance of yellow bands with the chromogenic reagent NP/PEG. Among the samples evaluated with the Folin-Ciocalteau reagent, the ethyl acetate and dichloromethane fractions had the highest total phenol content, 785.31 mg EAG/g and 132.09 mg EAG/g, respectively. CONCLUSION: The preliminary results show that phenolic substances in S. trilobata may be involved with the phytotoxic activity. The future objective is to continue the phytochemical investigation by biomonitoring the bioactive fractions, as well as to test them on weed seeds.