ABSTRACTThis work reports the evaluation of allelochemicals present in the essential oil of Cynophalla flexuosa under the germination and growth of Calotropis procera. The chemical composition was determined by gas chromatography coupled to mass spectrometry (GC/MS). The allelopathic potential of the oil was evaluated in C. procera. The data were subjected to a one‐way analysis of variance, and the means were compared by Tukey's test, 5% probability (p < 0.05). The results obtained in the GC/MS analysis showed that the major compounds were butyl isothiocyanate (42.60%) and isobutyl isothiocyanate (42.28%). The germination speed index (GSI) showed negative results for all treatments evaluated, with greater significance for higher concentrations. The different concentrations of the oil caused negative results in the average growth of the radicle and stem of C. procera. Additional studies on the compounds responsible for the observed activities and new tests to confirm the use of natural herbicides are essential to ensure their use.