A scanning underwater, ultrasonic system has been developed to measure the in vivo vibrational amplitude response of fish hearing organs to low-frequency sound. The fish is insonified with a 20-MHz source while stimulated in a single-frequency sound field. Spectral analysis of the echoes due to the acoustic impedance mismatches at the auditory organs provides the desired amplitude information. A computer-controlled scanning system allows nearly simultaneous measurements of the response of the various organs. The system is capable of measuring displacements of 12Å with a spatial resolution of 0.28 mm.