The low temperature magnetoresistance of a high mobility two-dimensional electron gas - realized in GaAs-AlGaAs heterojunctions - is dominated by Shubnikov-de Haas oscillations, reflecting the discrete nature of the electron energy spectrum. When a weak one- or two-dimensional periodic potential with period a smaller than the electron mean free path is superimposed on the two-dimensional electron gas a novel type of oscillation occurs which reflects the commensurability of the relevant lengths in these systems - the cyclotron orbit diameter at the Fermi energy and the period a of the periodic potential. Theoretically, this lateral superlattice effect is shown to result from an oscillatory dependence of the bandwidth of the modulation-broadened Landau levels on the band index.