Preflocculated ferric hydroxide flocs were subjected to either a simple shear flow or a two-dimensional straining flow, and their motion was optically observed. Digital image analysis was applied to extract information on orientation and deformation from the digitized frames. It was found that the simple shear flow led to a rotation of the flocs whose motion can be understood from the behavior of a solid ellipsoid. In the extensional flow, no continuous rotation occurred and flocs were broken apart along the axis of straining. The rupture forces estimated from an ellipsoid model were found to be in the range of 0.1 N/m(2).