In a Gunn effect oscillator operating in the transit‐time mode, domains of high electric field are formed periodically and travel through the device. When such an oscillator is placed close to the surface of a piezoelectric solid the moving high‐field domains produce moving local stress concentrations in the piezoelectric, causing the radiation of elastic waves into the body of the piezoelectric and along its surface. The domain velocity is much higher than the elastic wave velocities, so the situation is similar to that of Cherenkov radiation, and the waves are emitted at a large angle to the direction of domain motion. Results are given on the generation of bulk and surface waves at a fundamental frequency of 114 MHz in a lithium niobate crystal coupled to a gallium arsenide Gunn effect oscillator.