To determine the sound velocity in wurtzite AlxGa1-xN, we have used surface acoustic-wave (SAW) delay lines on AlxGa1-xN/c-Al2O3. AlxGa1-xN films with compositions from x=0 to x=1 were grown by plasma-induced molecular beam epitaxy. Starting from published data, we fine tuned the values of the elastic moduli used in numerical calculations such that the simulated and measured dispersion of the SAW were in good agreement. Based on these values, the surface and bulk acoustic-wave velocities of single-crystal AlxGa1-xN were determined as functions of the composition. The resulting SAW velocities ranged from 3700 to 5760 m/s for GaN and AlN, respectively. © 1998 American Institute of Physics.