Application of the Darwin‐Prins theory to diffraction by a perfect noncentrosymmetric crystal shows that the ratio of integrated intensities for opposite faces is given by a relationship that is identical to that for a mosaic crystal.I(h,k,l)/I(h¯,k¯,l¯)=|F(h,k,l)|2/|F(h¯,k¯,l¯)|2.This result had been derived previously from the Ewald‐von Laue theory. The relationship has been verified experimentally with measurements on nearly perfect crystals of GaSb and InAs.