The theory of the internal strain, which has been verified experimentally for crystals with the diamond structure, is extended here to the zincblende structure. X-ray intensity measurements were taken on GaAs under a uniaxial stress along the [111] direction. It was found that this stress causes a slight change of the structure factor of the weak reflexion 006, from which the value ζ = 0.046 ± 0.002 for the bond-bending constant was estimated. This means that in GaAs the effect of the internal strain is very weak, so that the tetrahedral bond changes its length almost according to the macroscopic strain tensor.