An appreciable increase in the dynamic magnetostriction was observed in polycrystalline partially aluminum-substituted yttrium iron garnets with small quantities (up to 0.01 mol.%) of implanted divalent cobalt ions. The nonmonotonic behavior of the concentration dependence of the magnetoelastic interaction as a function of the cobalt ion content is explained with allowance for changes in the anisotropy fields and the profile of the magnetization curve in doped samples.