Normally consolidated samples of kaolin have been tested in two direct simple shear apparatuses. The first apparatus is specially instrumented with load cells in the horizontal boundaries to investigate the uniformity of the stresses acting on the samples. The internal deformations and strains are determined using radiographic techniques. The radial stress is determined from membranes reinforced with resistance wire. The second apparatus is a standard commercially available device supplied by Geonor. The stresses and strains are shown to be uniform in the central region of the sample, away from the vertical boundaries, until failure. Values of the stress and strain variables determined from the central region are representative of a uniform simple shear deformation in plane strain. In all tests the samples fail by developing approximately horizontal ruptures. Values of the shear modulus and shear strength estimated from the average stresses in the Geonor apparatus underestimate, by approximately 10%, the simple shear values.