Ultrasonic testing of castings is difficult due to the influence of the casting process, microstructural variations, surface roughness and the complex shape of castings. The use of ultrasound for the testing of castings depends on the skill and expertise of the operators. However, the initial experimental setup is critical in developing a proper test procedure. This paper describes the procedure used to determine a suitable frequency range for the testing of UNS A23800 aluminum alloy castings while accommodating material variations within the part. The results obtained from immersed pulse/echo ultrasonic testing indicate that focused probes operating at frequencies between 5 and 10 MHz are more suited to the testing of aluminum alloy die castings with surface roughness R[a] values varying from 50 to 100 μm (2 × 10[-3]to 4 x 10[-3] in.). Nevertheless the final selection of a suitable frequency will depend largely on the size of discontinuities to be detected.