Several types of poly(Hyp(Bzl)-co-Pro)s were synthesized. Their molecular conformations and thermosensitive properties were investigated in an attempt to find new thermosensitive materials. These polypeptides assumed the polyproline-II structure in the temperature range of 20 to 80degreesC. They also exhibited cloud points in light transmittance, indicating the phase transition. The transition temperatures decreased with increasing hydrophobicity of the polypeptide.