A new type of magnetoresistance oscillation periodic in 1/B is observed when the carrier density Ns of a two-dimensional electron gas is weakly modulated with a period smaller than the mean free path of the electrons. Experiments with high mobility AlGaAs-GaAs heterojunctions where Ns is modulated by holographic illumination at T ¡Ü 4.2 K show that the period of the additional quantum oscillation is determined by the separation a of the interference fringes. This period corresponds to Shubnikov-de Haas oscillations where only the electrons within the first reduced Brillouin zone with |k| ¦Ð/a contribute.