The shear stress τ–shear strain γ behavior of suspensions of carbonyl iron particles in silicone oil was determined at a shear rate γ̇=0.042 s−1as a function of applied magnetic inductionB0(0.5–6.0 kG), volume fraction of particles φ(0.05–0.30), and roughness of the shearing surfaces. At low fields, the steady‐state shear stress τSwas significantly higher for a roughened surface compared to a smooth surface, but became only slightly dependent on roughness at high fields. In accord with theoretical considerations, the ratio τS/φ was proportional to the square of the calculated magnetic fieldH0inside the suspension at low fields, but its dependence onH0became smaller as the field increased. The smaller dependence may result from the magnetic saturation of the particles or the slipping boundary condition.