We report the observation of Landau ladders attached to each Stark transition in GaAs/Ga0.65Al0.35As superlattices under magnetic and electric fields parallel to the growth axis, z. For certain ratios of magnetic and electric fields, anticrossings occur between Landau ladders associated with different Stark levels. The coupling between the Landau ladders is ascribed to the excitonic interaction and the valence-band mixing. For an increasing magnetic field perpendicular to the electric field (Bz), the excitonic Stark transitions broaden and their intensities decrease until they eventually vanish. This behavior can be explained in terms of a competition between magnetic-field- and electric-field-induced localizations. © 1991 The American Physical Society.