By means of inelastic neutron scattering we have measured the phonon dispersion curves in the main symmetry directions of the cubic Laves phases YAl2 and LaAl2. The phonon frequencies in LaAl2 are considerably softer than those of YAl2 which cannot be explained solely by the different atomic masses of La and Y. The experimental dispersion curves are well described by axially symmetric Born v. Kármán models taking into account interactions up t o the fifth nearest neighbors. From this analysis we find that the nearest neighbor Al-Al and TM-TM longitudinal force-constants of YAl2 are about twice those of LaAl2 whereas the TM-Al interactions are nearly equal