Surface tension, small angle neutron scattering (SANS), freeze-fracture transmission electron microscopy (IFF-TEM), and circular dichroism (CD) have been used to study the self-aggregation properties of the natural lipopeptide [Glu(1), ASP(5)] surfactin-C15 in 0.01 M phosphate buffer Solution (PBS) at pH 7.4. It has been found that the critical micelle concentration (cmc) of surfactin is 1.54 x 10(-5) M, the surface tension at the cmc (sigma(cmc)) is 27.7 mN/m, and the area per molecule at the air-water interface is 107.8 angstrom(2). Surfactin molecules adopt a beta-sheet conformation already at low concentrations. This feature probably makes it surface-active at Such low concentrations. From SANS and FF-TEM results, it is seen that surfactin exhibits a strong self-assembly ability to form sphere-like micelles and some larger aggregates even at the rare low concentration. The aggregation number of sphere-like micelles is much smaller than that for conventional surfactants of similar alkyl chain length.