description of the subject. essential oils are volatile fractions produced through the special metabolism of plants. one significant biological application of essential oils in recent years has been their use as bioherbicides, due to their important biocide effect. objectives. this study aims to identify the chemical constituents of aloysia gratissima (gillies & hook.) tronc. essential oil and assess its biocide potential. method. this study describes, for the first time, the chemical constitution of the essential oils from seeds of a. gratissima. to verify possible biocide effect, lettuce seeds were submitted to different concentrations of essential oils from a. gratissima leaves (ag-le-eo), flowers (ag-fl-eo), and seeds (ag-se-eo) to evaluate germination percentage, mean germination time, synchrony of germination, and seedling length. results. at concentrations of 800 l.l-1, synchrony was positively influenced by (ag-le-eo), increasing from 0.38 to 0.54. on the other hand, ag-se-eo at concentrations of 800 l.l-1 reduced lettuce germination from 90.8% to 64.4% and seedling length from 1.35 to 0.8 cm, besides increasing mean germination time, indicating biocide effects. conclusions. these results are expected to lead to further trials to understand how the constituents of a. gratissima essential oils inhibit germination, making them a potential bioherbicide.