The piezobirefringence of GaAs has been measured over the wavelength range 3.5–10.6 &mgr;m. A small yet significant dispersion is found which is attributed to the strain dependence of the transverse optic phonon. The main contribution to the dispersion appears to be due to the strain‐induced anisotropy of the transverse effective charge. The data are in reasonably good agreement with the theory of Humphreys and Maradudin. The strain‐induced relative anisotropies of the transverse effective‐charge and the high‐frequency photoelastic constantsk∞11−k∞12andk∞44have been calculated on the basis of a two‐parameter fit to the photoelastic dispersion.