The retentive force of a wire clasp composed of a b-type Ti-6Mo-4Sn alloy (b-Ti alloy) with a low Young modulus was evaluated using a piezoelectric transducer to determine the appropriate undercut for removable partial dentures. There were no significant differences in retentive force between a b-Ti alloy wire with a 0.50-mm undercut and a cobalt-chromium alloy (Co-Cr alloy) wire with a 0.25-mm undercut, or between a b-Ti alloy wire with a 0.75-mm undercut and a Co-Cr alloy wire with a 0.50-mm undercut. The b-Ti alloy wire may be applicable for abutment teeth with a large number of undercuts. Int J Prosthodont 2010;23:38–41.