We have calculated the binding energy and the transition energies of excitons for a superlattice under an external electric field applied along the growth direction. At low fields the binding energy shifts strongly showing the localization of the superlattice states into the Stark ladder states. For higher fields the energy levels observe a shift associated to the single quantum well Stark effect. We also discuss the contribution of superlattice fluctuations to the carrier confinement. © 1990.