The Tb₅(Si_xGe_4−x) alloy system is similar to the better known Gd₅(Si_xGe_4−x), except it has a more complex magnetic and structural phase diagram. Gd₅(Si_xGe_1−x)₄ has received much attention recently due to its giant magnetocaloric effect, colossal magnetostriction and giant magnetoresistance in the vicinity of a first order combined magnetic-structural phase transition. The magnetostriction changes that accompany the phase transitions of single crystal Tb₅(Si_2.2Ge_1.8) have been investigated at temperatures between 20 and150 K by measurements of magnetostriction along the a axis. Over this temperature range the shape and slope of the magnetostriction curves change, indicative of changes in the magnetic state, crystal structure, and magnetic anisotropy. The results appear to indicate a phase transition that occurs near 106 K (onset-completion range of 116–100 K). The steepness of the strain transition, its unusual hysteresis, and its temperature dependence appear to indicate a first order phase transition which is activated by applied magnetic field in addition to temperature (see Fig. 1). Magnetostriction measurements at temperature below the transition region appear to indicate a magnetostriction of small overall magnitude (about 30×10⁻⁶) but high anisotropy, with anistropy showing considerable temperature dependence.