Abstract: Cestrum parqui is a shrub originating from Chile used in Tunisia as an ornamental plant. The toxicity of this plant was observed for the first time on the Desert Locust Schistocerca gregaria. It was shown that the toxicity of the plant is concentrated in the Crude Saponic Extract (CSE). In the present research we try to study chemically this fraction to isolate the active substances. The CSE was separated by column chromatography and the obtained fractions were biologically tested on Schistocerca larvae. We observed that only the fraction F9 was active, the separation of this fraction on preparative plates permits to isolate the active compound called S1. The tentative of structure elucidation of the natural substance S1 using 1H and 13C NMR shows that S1 is an heterosidic saponin. Structural modification of S1 structure provoke its activity lose.