An emulsion polymerization of styrene in the presence of an amphoteric emulsifier of amino-acid type, N-lauroyl-N,N-dimethyllysine (DMLL), has been studied at pH 7.0. The polymerization rate increased exponentially with the concentration of DMLL. The concentration of latex particles was proportional to the DMLL concentration to the 0.6 power. The latex particle size decreased with increasing concentration of DMLL. The molecular weight of polystyrene latexes increased with an increase in DMLL concentration. Furthermore, the zeta potentials of latex particles were significantly dependent on the pH, and showed the existence of an isoelectric point at about pH 4.0.