The standard theory of magnetocrystalline anisotropy for a rigid ferromagnetic body, with cubic symmetry, makes use of a ''zero strain'' anisotropy constant K1. If the effect of magnetostriction is taken into account, this constant must be replaced by a ''zero stress'' anisotropy constant K'1 which differs from K1 by a term involving magnetostriction constants and elastic constants. There are also zero strain and zero stress versions of higher order constants (K2, etc) for cubic symmetry, as well as for other symmetries. The constant K'1 also appears in dynamic behavior of ferromagnets driven by an applied field, unless the field changes too rapidly for the system to remain in equilibrium. It is predicted that the ferromagnetic resonance (FMR) experiments give a direct measurement of K1. ¿ American Geophysical Union 1994