Two thermal methods capable of determining time averaged, total radiated ultrasonic power at diagnostic levels are described. The first, a portable instrument, utilizes a parabolic reflector to focus the ultrasonic beam onto a thin film thermistor sensing element mounted on sound absorbing rubber. The sensor's temperature rise alters its resistance, which is monitored with a simple bridge circuit. Maximum sensitivity of the device is 0.1 mW and beams up to 19 mm diameter can be accomodated. Calibration was performed using 1 MHz, 2.25 MHz, 3.5 MHz, 5 MHz and 10 MHz resonant transducers. The second device, a twin, series flow, ultrasonic comparator, is a primary measurement laboratory system. Ultrasound is absorbed by a series of rubber wafers within a vessel and is compared with measured dc electrical power dissipated in a second, identical vessel. The heat produced within the vessels is carried to twin, resistance thermometer temperature sensors by an inert perfluorinated liquid, within a closed circuit. In automatic mode the bridge is coupled to a feedback circuit which applies electrical power to regain bridge balance. Unwanted heat produced within the transducer being tested is carried away by a flow of water between the transducer and absorbing vessel. This instrument can measure power from 0.2 MW to 10 W and can accomodate transducers resonant at 1 to 15 MHz and up to 26 mm in diameter.