The thixotropic behavior of a colloidal dispersion of clay consisting of disk-shaped particles was studied by means of a combination of rheometric measurements, static light scattering, and small-angle neutron scattering. At rest, the structure of the gel consists of dense micrometer-sized aggregates assembled into a fractal mass of dimension D. Under shear, in the case of volume fractions with (1⩽D⩽1.2) at rest, a butterfly-type light scattering pattern is observed. This is attributed to the formation of rollers within the dispersion, which align themselves on average perpendicular to the direction of shearing. This produces a fall in resistance to flow and in viscosity. The influence of shear rate on this disaggregation process was studied and linked to the rheometric measurements. Under shear flow conditions, the fall in viscosity is due to orientation and disaggregation processes occurring at length scales on the order of 1 μm. During recovery, the two time scales identified correspond, respectively, to a rapid relaxation of the particle orientations and a slow aggregation process. Thixotropic behavior is identified as being mainly a reversible aggregation process, for which large length scales on the order of a micrometer associated with a fractal arrangement play a decisive role. © 1998 Society of Rheology.