Cast aluminium-silicon, cast aluminium-zinc and mechanically alloyed aluminium alloys were eroded in distilled water using a 20 kHz ultrasonic vibratory device. The erosion was measured by weight loss, and the damaged surface was examined using metallographic and profilometric techniques. The maximum differences in the incubation period, in the linear erosion rate and in the mass loss after a 10 h exposure for the nine materials investigated were 620%, 740% and 250%, respectively. The mechanically alloyed materials had by far the best combination of erosion properties. The cast Al-Si alloys had the poorest resistance to erosion. Age hardening was particularly beneficial with the Al-Si alloy. The main mechanism of material removal in all the alloys is by plastic deformation and ductile fracture.