Holographic interferometry is a specific technique for the analysis of vibration. Images of vibrating body reconstructed with this method are superimposed by interference fringes. As the fringes represent iso-amplitude contours like those on geodesic maps, the mode of vibration is clearly discernible at a glance. As the amplitudes relating to each fringe are calibrated according to the wavelength of the laserlight the value is most precise and submicroscopic, viz., 10(-5)cm order. In this study eardrum vibrations are investigated by means of time-averaged holography on frogs and guinea-pigs. The first fringe appeared at about 80 dB sound pressure level in both animals. The vibratory pattern appeared to be constant depending upon the number of frequencies of the test tones. At low frequency the eardrum showed rather a simple pattern; with increasing frequencies the pattern became more complicated containing two or more mounds.