Magnetorheological suspensions (MRS) are suspensions of micronic magnetic particles which exhibit a transformation from a liquid-like structure to a gel like structure when the magnetic field is raised. The yield stress which is necessary to break this structure can be quite important (up to 100 KPa) for concentrated suspensions of particles with a high saturation magnetization “Ginder et al 1996”. The interest for applications is to be able to electronically control the yield stress and numerous devices have been proposed which use this effect to control a damping rate or the transmission of a torque or of a force. For these reasons the study of the rheology of MRS has often focused on the yield stress rather than on the dependence between the shear stress and the shear rate. In this paper we focus on the change of structure induced by the flow of a MRS in the presence of a magnetic field.