An investigation of the behavior of Plexiglas and polystyrene lenses as focusing devices for ultrasonic radiation is described. A fixed frequency of 1 megacycle/second and intensities up to 6.6 W/cm2were employed. Using a thermocouple probe to obtain records of intensity of radiation in a water tank, it was found that above about 2 W/cm2the Plexiglas showed signs of physical breakdown. Up to the maximum intensities employed, polystyrene showed no indication of damage, while at 6.6 W/cm2the Plexiglas developed internal cavities near the outer edge of the lens, where the thickness was 4 mm.