An Underground Research Laboratory (URL) has been constructed in the deposit of Boom clay at a depth of 223 m by the SCK-CEN Belgian organisation near the city of Mol. This URL is devoted to research into nuclear waste disposal. This paper presents the results of an investigation carried out in the triaxial apparatus on specimens that were trimmed from blocks extracted during excavation sequences in the URL. In order to characterize the mechanical behaviour of the natural Boom clay and to examine the effect of initial and induced anisotropy on its constitutive behaviour, two series of triaxial tests were carried out. Special attention was devoted to the yield behaviour and the effects of stress history. Experimental results showed a clear relation between the shape of yield curves and stress history. The yield curve of the clay in its initial state was oriented along the K0 line, illustrating the anisotropy of fabric that was generated during the soil deposition. Subsequent isotropic compression (up to 9 MPa) made the yield curve more and more oriented along the p' axis. Based on these experimental results, an elasto-plastic model accounting for isotropic and anisotropic hardening in (p':q) space was developed. The initial yield curve of the soil was taken inclined with respect to the p' axis. The formulation proposed describes the change in shape, size and orientation of the yield curve, according to the stress history. Eight constitutive parameters were used to describe the anisotropic behaviour of the soil.