The large-angle convergent-beam electron diffraction technique has been used to obtain 200 rocking curves from a l500 Å Al,0·4Ga0·6As/30Å GaAs/3500Å Al0·4Ga0·6As sample observed in plan view. It is demonstrated that the rocking curves are sensitive to the presence of the GaAs layer and are in good agreement with simulations calculated from a simple kinematical approach. It is shown that both the layer thickness and depth in the foil may be examined and that GaAs layer thicknesses less than 10 Å in the 5000 Å foil may be detectable.