It has been shown that the membrane motor in the outer hair cell is driven by the membrane potential. Here we examine whether the motility satisfies the reciprocal relationship, the characteristic of piezoelectricity, by measuring charge displacement induced by stretching the cell with known force. The efficiency of inducing charge displacement was membrane potential dependent. The maximum efficiency of inducing charge displacement by force was approximately 20 fC/nN for 50-microm-long lateral membrane. The efficiency per cell stretching was 0.1 pC/microm. We found that these values are consistent with the reciprocal relationship based on the voltage sensitivity of approximately 20 nm/mV for 50-microm-long cell and force production of 0.1 nN/mV by the cell. We can thus conclude that the membrane motor in the outer hair cell satisfies a necessary condition for piezoelectricity and that the hair cell's piezoelectric coefficient of 20 fC/nN is four orders of magnitude greater than the best man-made material.