[en] We have studied the dispersion of long wavelength longitudinal phonons in silicon and germanium using ultrasonic techniques. For long wavelengths, the acoustic phonon dispersion relation is of the form ω(k)≅ck-γk³ , where c is the speed of sound and γ measures the lowest-order phonon dispersion. By sending an ultrasonic pulse of length a few hundred angstroms into a crystal and measuring the change of the pulse shape with propagation distance, we are able to determine the parameter γ . The results are compared with lattice dynamics models. (c) 2000 The American Physical Society