Measurements have been made of the temperature dependence at 1 GHz of the ultrasonic attenuation of various modes in single crystals of n-type indium antimonide and gallium arsenide. The observed attenuation has been compared with predictions based on the work of Simons and Maris which takes into account the finite lifetime of the thermal phonons with which the ultrasonic phonons interact. The agreement obtained is in general acceptable, considering that in the treatment given, there are no adjustable parameters. Further experiments are suggested to identify the individual processes which are responsible for the attenuation. It is suggested that information concerning the lifetime of specific thermal modes may soon be available from such attenuation measurements.