Magnetic breakdown is observed in a two-dimensional electron system subject to a strong, atomically precise, one-dimensional potential with a period of 15 nm. The transition from closed to open electron orbits is studied in magnetotransport experiments by continuously changing the Fermi energy of the superlattice within and above the first miniband. Shubnikov-de Haas oscillations quench for Fermi energies close to the miniband gap but recover at higher magnetic fields. The density of states is clearly altered from a conventional 2D system which manifests itself in aperiodic magnetooscillations when sweeping the Fermi energy at fixed magnetic fields. (C) 2000 Elsevier Science B.V. All rights reserved.