The molecular mode of action of the lipopeptide SF with zwitterionic and negatively charged model membranes has been investigated with solid-state NMR, light scattering, and electron microscopy. It has been found that this acidic lipopeptide (negatively charged) induces a strong destabilization of negatively charged micrometer-scale liposomes, leading to the formation of small unilamellar vesicles of a few 10s of nanometers. This transformation is detected for very low doses of SF (Ri = 200) and is complete for Ri = 50. The phenomenon has been observed for several membrane mixtures containing phosphatidylglycerol or phosphatidylserine. The vesicularization is not observed when the lipid negative charges are neutralized and a cholesterol-like effect is then evidenced, i.e., increase of gel membrane dynamics and decrease of fluid membrane microfluidity. The mechanism for small vesicle formation thus appears to be linked to severe changes in membrane curvature and could be described by a two-step action: 1), peptide insertion into membranes because of favorable van der Waals forces between the rather rigid cyclic and lipophilic part of SF and lipid chains and 2), electrostatic repulsion between like charges borne by lipid headgroups and the negatively charged SF amino acids. This might provide the basis for a novel mode of action of negatively charged lipopeptides.