Recent experimental and theoretical results indicate that optical phonons in a GaAs/Ga_1-xAl_xAs heterojunctions show strong spatial modulation. The form of the modulation has been controversial. However, it has recently been shown that the scalar potential of the electric field associated with the phonons is well described by the dielectric continuum model. The authors present the results of the first calculation of electron mobility in a GaAs/Ga_1-xAl_xAs superlattice which takes into account the confinement of both electrons and optical phonons. The electron mobility for a low-density nondegenerate electron gas is evaluated by iterative numerical solution of the linearised Boltzmann equation. The modulation of the phonons reduces the scattering to 50% of that predicted using bulk phonons and the predicted mobility is significantly increased.