A new experimental technique is presented that combines electron microscopy of dislocation structures in metal crystals with the application of high-amplitude ultrasonic stress waves at energy densities between 105 and 1010 erg/cm3 at a nominal frequency of 40 kc/sec [B. Langenecker, AIAA J. 1, 80 (1963)]. The acoustic field is produced by a piezoelectric transducer and an ultrasonic-velocity transformer [E. A. Neppiras, Brit. J. Appl. Phys. 11, 143 (1960)]; it is observed by a very small magnetostrictive probe [B. Langenecker, J. Acoust. Soc. Am. 38, 234–238 (1965).] High-amplitude ultrasound is used to cause changes in dislocation structures, and the damping of the sound waves is measured as these structural changes occur. Preliminary results from these in situ observations carried out in a Siemens Elmiskop I are discussed in the light of present theories on internal friction in metal crystals.