Magnetostriction at room temperature under various conditions of compressive prestress and applied fields of TbyDy1−y(Fe1−xTx)2 (T=Al, Mn) twin-free single crystals was investigated. The substitution of Al or Mn for Fe lowers the magnetostriction under ordinary temperature and pressure, and decreases the saturation field, which enables these materials with potential benefits for applications. Moreover, Tb0.5Dy0.5(Fe0.9Mn0.1)2 shows negative magnetostriction at room temperature under zero prestress, due to the rotation of domains 109.5° away from 〈111〉. Under appropriate compressive stress, a quite large magnetostriction of 2160 ppm with a saturation field of 900 Oe and high d33 of 4.8 ppm/Oe can be obtained. © 1998 American Institute of Physics.