A corrosion fatigue testing system has been developed for examining the corrosion-fatigue strength of engineering materials in the very high cycle regime (10⁹ to 10¹⁰ cycles) typical of many design lifetimes. Central to this resonant fatigue testing system is a high efficiency, large displacement, piezoelectric transducer which operates at a frequency of 20 kHz, thus enabling the accumulation of large numbers of cycles in a reasonable amount of time. A description of apparatus, environmental controls, and testing method is presented. Experimental 20 kHz and low frequency corrosion fatigue results for a number of engineering materials, including a 12% chromium stainless steel, show similar reductions in fatigue limit at a specified number of cycles, despite the difference in frequency. This result holds for a variety of aqueous environments with and without electrochemical controls imposed during testing. The data illustrate the capabilities of the ultrasonic corrosion-fatigue technique and are analyzed to assess its advantages and limitations.