A mesoscopic model of a diblock copolymer is used to study the stability of
a lamellar structure under a uniform shear flow. We first obtain the nonli
near lamellar solutions under both steady and oscillatory shear flows. Regi
ons of existence of these solutions are determined as a function of the par
ameters of the model and of the flow. Finally, we address the stability of
the lamellar solution against long-wavelength perturbations.