A comparison is made of microstructural changes leading to fatigue of copper and alpha‐brass at small strain amplitudes imposed at the normal test frequency of 1700 cpm and at the ultrasonic frequency of 17 000 Hz. A striking difference appears in the distribution of slip. At normal frequency the slip spreads densely over a grain, with no slip‐band cracking (stage S of the S‐N curve). At ultrasonic frequency the slip concentrates in single isolated bands in only occasional grains and reduces these to microcracks before any general spread of slip can be observed. However, as might be expected, the ultrasonic slip did spread in specimens tested at elevated temperatures, which expedited dislocation climb and diffusion processes; then this spreading of slip prevented the slip‐band microcracking at ultrasonic frequency also. The internal friction for the copper was so high that fatigue cracks could not be generated even at room temperature.