A study was made of the magnetoelastic properties of a system of yttrium iron garnet crystals doped with Co**2** plus ions. Two methods were used: ferromagnetic resonance and static capacitance. All the investigated compositions exhibited ″anomalies″ . The results were used to calculate the temperature dependences of five magnetostriction constants. The results of these two methods were compared and analyzed on the basis of a microscopic theory.