We have performed lateral diffusion measurements of photo‐excited electron‐hole pairs in a GaAs/AlAs multiple quantum well structure under magnetic fields applied perpendicular to the wells, by using an all‐optical time‐of‐flight technique with a single‐mode optical fiber probe. Remarkable suppression of exciton diffusion is observed, and is ascribed to the magnetic perturbation by which moving excitons are affected proportionally to the square of wave vectors. ©1995 American Institute of Physics.