High‐power ultrasonics has been a useful tool in studying fatigue in metals, since a large number of cycles can be obtained in a small time. In the case of brass and iron, the mechanisms appear to be different, since at the high frequencies fatigue occurs in isolated regions, whereas at the low frequencies the slip is generally spread over the grains. The same mechanism can be produced at high frequencies by going to higher temperatures. In iron, the stress required for fatigue at higher frequencies is considerably less than for low frequencies. Recent measurements in titanium have shown that the required stress is about the same for high and low frequencies. The crack propagation velocity, however, is much faster at the ultrasonic frequencies. This was proved by striation measurements with a scanning electron microscope. A striation reflects the fact that the crack propagates under a different stress on one‐half of the cycle than it does on the other half. Hence, by counting the number of striations, the velocity can be calibrated; thus the high‐frequency fracture is not only localized, it is virtually catastropic in the localized area. [Supported by the Fatigue and Structural Reliability Institute under a contract with the U. S. Navy Office of Naval Research.]