A group theoretical description is given of the first and second order magnetoelastic coupling for cubic ferromagnets. First, this coupling is expanded as a series of the Lagrangian tensor componentsηij, which definesN coefficients. An alternative expansion is given in terms of the symmetrical (eij) and antisymmetrical (ωij) components of the homogeneous strainsuij, which definesN(>N) coefficients. The conditions for rotational invariance of the energy are then written: they provide relations which reduce fromN toN the number of independent coefficients in the 2nd description. These relations were previously derived in the framework of the localised model and some features of this model are briefly discussed here: in particular, the exchange contributions to the elastic constants are corrected for volume effects and differ significantly from Fuch's and Sato's formulae. In the next step, the magnetostriction is taken into account to derive the internal energy of a distorted crystal. It is then possible to analyse the ultrasonic waves propagation under high magnetic fields in terms of three main magnetic contributions, namely an isotropic exchange effect, an anisotropic morphic effect and a field dependent Simon effect.