The femora of thirteen dogs were exposed to high levels of ultrasonic energy. Alterations became evident on roentgenograms two weeks after such exposure. Death of cortical bone was evident histologically three and four weeks after exposure, the shortest time interval after exposure that bone was studied histologically. The periosteal and the endosteal responses proximal and distal to the area of necrosis consisted of periosteal and endosteal formation of new bone with a marked increase of the periosteal and endosteal vessels. These changes are interpreted as being secondary to the death of the segment of diaphyscal bone exposed to ultrasonic energy. Whether the death so produced is purely thermal or is due also to non-thermal effects is not known to us at this time. That ultrasonic energy can be focally destructive to bone and its blood supply seems apparent.