A comparative study of the undrained creep rupture characteristics of a saturated, normally consolidated, undisturbed marine clay has been carried out under triaxial and plane strain conditions. Creep rupture tests were performed on samples which were consolidated both isotropically and under K0 conditions to the same vertical effective stress. It is shown that for a given minimum creep rate or creep stress, large differences in rupture life are observed between conventional isotropically consolidated triaxial samples and K0 consolidated triaxial or plane strain samples. Similar discrepancies are introduced when conventional triaxial results are used to estimate remaining rupture life during tertiary creep stage when the real situation corresponds to either K0 consolidation or plane strain. Effective stress failure in creep rupture for a given type of test is governed by the same linear failure envelope which is obtained during constant rate of shear testing.