We have made magnetotransport measurements on two-dimensional electronic systems confined to AlAs by AlxGa1-xAs. We have determined the effective mass (m111=0.49m0), the effective g factor (3.1), and the relaxation time. Even though the lattice constants of GaAs and AlAs are nearly equal, the stress arising from this small mismatch strongly influences the electronic properties of AlAs. For -oriented samples the valley degeneracy is one, and there is an anomalous phase shift in the Shubnikovde Haas oscillations. © 1987 The American Physical Society.