A theory for the elastic properties of magnetostrictive materials is presented. The complete elastic constant tensor is obtained from simple linear magnetoelastic approximations as a function of the magnetoelastic coupling coefficients and the susceptibility. It is also shown that, by using the proper dynamic susceptibility tensor, most of the observed effects can be described. More specifically the field dependence of the attenuation and the difference in the ultrasonic shear-wave velocity between the cases of H\ensuremath{\rightarrow} parallel to the polarization and that of H\ensuremath{\rightarrow} parallel to the propagation direction are explained. A very good agreement is found when the theoretical predictions are compared with some available experimental data.