A novel, portable radiometer has been developed for routine calibration of the time-averaged ultrasonic power radiated from ultrasonic therapy devices. The radiometer combines a precision electrobalance with a reflecting target. The balance was calibrated at 5 watts at 1 MHz and at 1 watt at 1.6 and 3.7 MHz. The useful ranges of frequency and power were estimated to be from 0.8 to 4 MHz and 0.5 to 5 watts, respectively.