Ultrasonic attenuation and velocity measurements have been made in pure germanium and silicon (doping<1014impurity atoms per cc) and in dopedn‐type germanium andp‐type silicon. The attenuation in pure materials shows a continuous decrease as the temperature is decreased and a very low attenuation below 20°K. These results indicate that the energy losses are accounted for entirely by phonon‐phonon interactions. A calculation has been made of these losses, using a model based on the Akheiser effect and incorporating the recently measured third‐order elastic moduli of silicon and germanium. For both materials, the calculated values predict correctly the large difference between longitudinal and shear waves and agree quantitatively within 50% with the measured values over the whole temperature range.