Enantiomer-selective polymerization of (RS)-(phenoxymethyl)thiirane (RS-1) was carried out with ZnEt2/L-alpha-amino acid as an initiator system, and the effect of the initiator system on the enantiomer selectivity was examined with various amino acids. All polymerizations heterogeneously proceeded, and every initiating system was effective in producing optically active polymers. For the polymerization of RS-1 with diethylzinc (ZnEt2)/(L)-leucine (1/1), the conversion was 43.7% in 12 days, and the number-average molecular weight of the polymer was 18,000. The enantiomer selectivity was maximum when the molar ratio of the two components in the ZnEt2/(L)-alpha-amino acid system was 1:1. When the ZnEt2/(L)-leucine (1/1) system was used in the polymerization, the best result was obtained with an enantiomer-selectivity value of 5.36. During the polymerization, the S enantiomer was preferentially consumed, and the isotactic-rich polymer was enriched in the S configurational units produced.