One class of bioactive lipopeptides produced by Bacillus subtilis, fengycin, has strong surfaceactivity and interesting antifungal property with low hemolytic activityTo gain knowledge about the mechanism of action of fengycin onto biological membranes, it is essential to have a better understanding of its penetrating process and its molecular organization within a bilayer. Therefore, we propose to perform neutron reflectivity experiments by using perdeuterated phospholipid chains. This will allow us to study the degree of penetration into the phospholipids bilayers. Two kinds of phospholipid will be used, DPPC and POPC, in order to mimic rigid and fluid domains, respectively, of natural membranes.