The theory of water flow in swelling clays may be developed in a space coordinate system at rest, assuming that the flux of clay particles can be defined by the space gradients of potential responsible for the movement of clay particles, which is related to the gradients of water potential. Within this framework, the hydraulic conductivity of saturated and unsaturated swelling clays is defined and measured from the water and clay content profiles obtained from infiltration experiments into unsaturated bentonite. As a result, the hydraulic conductivity is found to increase from 10−7to 10−2cm/d when the volumetric water content increases from 0.05 to 0.95 cm3/cm3. These results agreed well with those analyzed by the existing theory, which was developed in the material coordinate system.