Deformation of the surface of a soft solid by an ultrasonically vibrating object (20 kHz) was studied by means of a displacement transducer. An ultrasonic horn with a suitably formed tip was vibrated at amplitudes of 1 to 25μ against agar gel, wax, plastics, and soft metals while penetration into the test sample was recorded as a function of time. As an interesting example, a hemispherically shaped vibrating tip of 0.1‐mm radius of curvature penetrated into paraffin wax in two successive stages—fast and slow. After initial penetration due to loading, the sound was switched on and the penetration rate changed abruptly from zero to 500 μ/sec. After 40 msec, the fast rate then changed to a slower one that gradually decreased to zero. Somewhat similar results were obtained for other solids such as agar gel, tin, lead, and plastic. Sonic penetration may arise from high shearing stresses near the tip, which lead to plastic flow. Currently these stresses are being studied in plastics by means of stress birefringence. Findings may increase understanding of effects of ultrasound on solid like media in general. [Research supported in part by Hood Interdisciplinary Fellowship.]