We report, for the first time, observations of the acoustic harmonics generated by steady‐state vibration at amplitudes so large that long‐range dislocation motion occurs. Most of the measurements have been made on the second and third harmonicsA2andA3. At low amplitudes the experimental results of Hikata and Elbaumetal. are confirmed, but there is some discrepancy with theory forA3. At amplitudes such that dislocation sources operate, the harmonic spectrum departs strongly from the Hikata‐Elbaum predictions. The harmonic amplitudes do not always increase monotonically with strain;A2occasionally shows a maximum,A3sometimes, andA5even more frequently, in a pattern sensitive to mechanical history. The spectrum depends on grain size and prestress, varying jointly with the modulus defect and internal friction as these indicators of dislocation motion vary. The results on strain dependence are in accord with a simplified model of dislocation breakaway and Frank‐Read source operation proposed previously to account for the internal friction and modulus defect.