Single interface modulation‐doped Alx Ga1−xAs /GaAs heterostructures with the binary on top of ternary were grown by molecular beam epitaxy. By incorporating a 150‐Å‐thick Alx Ga1−xAs /GaAs three‐period superlattice in place of an undoped Alx Ga1−xAs spacer layer, 10‐K mobilities of up to 256 000 cm2/Vs were obtained. This value is about 6.5 times that of the previous best value. This dramatic improvement is tentatively attributed to the relief of strain caused by the small but significant lattice mismatch although impurity trapping by the superlattice may also play a role. Normal modulation‐doped structures where the ternary is grown on top of binary also showed mobility improvement (about 30%) when the undoped AlGaAs spacer layer is replaced with a three‐period superlattice of the same thickness. This concept should have a significant role in heterojunction bipolar transistors, field‐effect transistors, lasers, and other heterojunction devices.