Poly(L-glutamic acid ester) with 3-(pentamethyldisiloxanyl) propyl groups was prepared from poly(gamma-methyl-L-glutamate) (PMLG) by ester exchange reaction. This polymer had an alpha-helical structure in the membrane prepared from 1,2-dichloroethane solution. The membrane forming ability of the polymer was better than that of PMLG. Optical resolution of (+/-)-tryptophan was achieved by pressure driven permeation through the membrane. The enantioselectivity was 16% enantiomeric excess (%ee) and the permeation rate was high (10(-6) g m/m2 h). This enantioselective permeation continued for more than 160 h. With an increase in the disiloxane side chain content, the permeation rate could be increased while maintaining the selectivity. Since the enantioselectivity for adsorption favoured the (+)-isomer and that for permeation favoured the (-)-isomer, enantioselective permeation was caused by suppression of the (+)-isomer permeation. It was suggested that the permeation of dilute aqueous tryptophan solution occurred mainly in the region of highly mobile disiloxane side chains, which had no asymmetric centers, by measurements and analysis of the pervaporation of dilute aqueous ethanol solution. Therefore it was believed that this siloxane region was small enough for the asymmetric centers in the main chain to recognize enantioselectively the permeating solute.