Relative sound output of bar magnetostrictors has been measured as a function of polarization at atmospheric stress and at a compressional stress of about 300 atm. Frequencies both at and well below the resonant frequency of the bar have been used. Thin laminations of nickel or permendur are glued together to form bars whose ends are then ground parallel. Each bar is length resonant at about 23 kcps. Half‐wavelength brass stubs with large flanges at their centers are soldered to the bar and loosely mounted in cylinders. Air can be forced between the flanges and cylinder walls to apply compressive stress to the bar. The displacement of the end of one of the stubs is observed. For nickel, compressive stress increases sound output at low polarizations and decreases sound output at higher polarizations. Representative values are these: a compressive stress of 300 atm increases the resonant amplitude by 15% at a polarizing field of 20 oe and decreases it 15% at 70 oe. The polarization at which 300 atm compressive stress does not alter the resonant displacement is 45.3 oe. Displacements at 60 cps and at resonance are related by a theory involving the magnetostrictive constant and the reversible permeability, and there is good agreement between this theory and experiments. (This research was supported by the Office of Naval Research.)