A physical similitude of the mechanical behavior of soils is investigated on the basis of a series of triaxial undrained CIU tests of kaolin. The deformation process is, as a rule, discontinuous as disclose a segmentlike form of the stress paths. Physically isomorphous behavior was identified only in a narrow range of stresses. This is explained by the sensitivity of the soil structure to reflect different stress states, stress being one of the most important state variables of soils.