Line‐focus beam acoustic microscopy has been used to study changes in the elastic constants of GaAs during amorphization produced by implantation with Si+ions at liquid‐nitrogen temperature. The distribution of amorphous material was determined by Rutherford backscattering and channeling. Values ofc11andc44were estimated by fitting theoretical curves to the measured angular dispersion of surface acoustic waves in the (001) plane. The implanted material was modeled as a statically stressed anisotropic layer on an unmodified GaAs substrate. The values ofc11andc44were found to decrease with increasing ion fluence. At the highest fluence the implanted region was completely amorphous, and it was observed that the softening ofc44(41%) was significantly greater than that ofc11(17%). ©1995 American Institute of Physics.