The allelopathic action can be verified by the presence of phytotoxic substances, molecules that are directed to the agribusiness sector. The objective of this study was to evaluate the chemical composition of the methanolic extracts from Colletotrichum dianesei and Xylaria sp., and phytotoxic potential on Lactuca sativa. The fungi were grown in Sabouraud liquid medium supplemented with 0.2% yeast extract, under stirring. The mycelia were subjected to solvent extraction in order of increasing polarity, but methanol extracts were used in this investigation. The allelopathic activity was evaluated at a concentration of 1000 µg/mL in quadruplicate, evaluating: percentage and speed of germination, and growth of lettuce seedlings under laboratory conditions. C. dianesei extract reduced germination percentage by 36% and delayed germination speed by 63.8%, while Xylaria sp. didn't interfere with the germination percentage, it only delayed the germination rate by 43.9%. Root growth was negatively affected in 30.5% and 51.1% under the influence of extracts from Xylaria sp. and C. dianesei, there was also a reduction of 33.1 and 31.8% in the aerial part length, respectively. The thin layer chromatography showed the presence of aromatic substances and terpenes in both extracts, and alkaloids only in C. dianesei extract. The 1H nmr spectra showed signals between 6 and 8.5 ppm (aromatic substances), being relatively more intense for Xylaria sp., signals between 3 and 4 ppm (sugars, showing anomers only for Xylaria sp.) and methyl groups between 0.7 and 1.3 ppm, suggesting terpenes. The results suggest that the fungal species evaluated have phytotoxic potential, and produce terpenes and aromatic substances.