Preconsolidated montmorillonite clay was deformed up to 30% strain at confining pressures up to 4 kb (400 Mpa) under undrained conditions. The deformation was characterized by plastic yield, strain hardening, a broad peak strength at large strain, and a gradual decline in strength thereafter. In some cases the sample was ruptured, with stress drop of the order of of 0.1 kb. At confining pressures greater than 2 kb, the peak strength was about 0.35 kb and did not depend markedly upon confining pressure. These results may have important implications on the stability and the strength of faults.