The elastic, piezoelectric, and dielectric constants of 0.58Pb(Mg1/3Nb2/3)O3-0.42PbTiO3 (PMN-42%PT) were determined experimentally using combined resonance and ultrasonic methods. At room temperature the PMN-42%PT single crystal has tetragonal symmetry. The measured piezoelectric constant d33 is ∼260×10-12 C/N. The electromechanical coefficients k15, k31, k33, and kt are 0.80, 0.39, 0.78, and 0.62, respectively. From the measured material constants the orientational dependence of phase velocities and electromechanical coupling coefficients was calculated. The results showed the tetragonal crystal exhibits isotropy in the X-Y plane and k15, k33, and kt reach their maxima in . © 2004 American Institute of Physics.