The frequency response of the dielectric constant(&egr;r), dielectric loss tangent (tan &dgr;) and impedance (Z) of an organic electro-optic crystal,(−)−2−&agr;- methylbenzylamino-5-nitropyridine (MBA-NP) monitored along the polar axis, is characterized by the presence of strong peaks in the kHz region. The observed resonance behavior, which is strongly dependent on the geometric size and shape of the sample, is attributed to its piezoelectric nature. The resonance peaks were found to shift steadily towards lower frequencies with increasing temperature and towards higher frequencies with increasing pressure. The temperature dependence of the stiffness coefficient(C)of the crystal has been defined using the resonance data. The temperature dependence of the damping of these piezoelectric vibrations has been monitored by the resonance–anti-resonance method. The influence of the application of pressure and the variation of temperature on&egr;r,monitored along the polar axis of the crystal, has been studied. ©1997 American Institute of Physics.