Emulsion polymerization of styrene was conducted using a biosurfactant (i.e., sodium surfactin, hereinafter 30 called just "surfactin") having very low critical micelle concentration (CMC, 2.9 X 10(-3) rnmol/L) and biodegradability characteristics. The nucleation mechanism was investigated by comparing with a conventional surfactant sodium dodecyl sulfate) system. Unlike the emulsion polymerization systems using conventional surfactants, nucleation mechanisms changed above CMC in the presence of a biosurfactant. At low concentrations of surfactin (above CMC), the polystyrene (PS) particles are likely generated via a soap-free emulsion polymerization mechanism. In contrast, at high surfactin concentrations, the PS particles would be synthesized by following a micellar nucleation mechanism. However, the slope (0.23) of the log N-p versus log C-s plot (N-p: number of particles; C-s: concentration of surfactin) did not obey the Smith Ewart theory (0.6), this probably being produced by the high adsorbability of surfactin.