Irradiation of strain‐free methacrylate with a single beam of focused ultrasound in adequate dosages results in the development of trackless focal alterations within the plastic. These alterations, easily visualized with polarized light, are egg‐shaped regions of stress and are probably thermal in origin. There is a consistent and reproducible relationship between ultrasonic dosage and the size of these “lesions,” comparable to that obtained when the brain of the cat is similarly irradiated. The use of methacrylate as a preliminary test medium permits rapid evaluation of the probable effects of varying dosage parameters as well as of the repetitive (and long term) stability of the ultrasonic output.