We have measured the magnetomechanical coupling in highly magnetostrictive TbxDy1-xFe2-δas a function of bias field, x, and δ. We find a peak coupling factor of 0.6 at x = .26 and bias field of 180 Oe; the largest reported for a magnetostrictive material. The corresponding material Q is 200. Because of the high magnetostriction, relative permeabilities are necessarily low; . Young's modulus at constant field changes from 4.5 to ll (x1010n/m2) as the bias changes from zero to 4.5 kOe, corresponding to a change in sound velocity >50%. The magnetomechanical coupling factor was also evaluated for . When δ is non-zero a eutectic second phase is present which acts as a toughening agent.