The piezoelectric properties of relaxor based ferroelectric single crystals, such asPb(Zn1/3Nb2/3)O3–PbTiO3andPb(Mg1/3Nb2/3)O3–PbTiO3were investigated for electromechanical actuators. In contrast to polycrystalline materials such asPb(Zr,Ti)O3,morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients(d33’s)>2500 pC/N and subsequent strain levels up to>0.6&percent;with minimal hysteresis were observed. Crystallographically, high strains are achieved for ⟨001⟩ oriented rhombohedral crystals, although ⟨111⟩ is the polar direction. Ultrahigh strain levels up to 1.7&percent;, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to anE-field induced phase transformation. High electromechanical coupling(k33)>90&percent;and low dielectric loss<1&percent;,along with large strain make these crystals promising candidates for high performance solid state actuators. ©1997 American Institute of Physics.