A new interpolation method called discrete smooth interpolation was used to construct a 3-D model of an activated sludge flee from digitized microtome sections. This method comprises several advantages : rapidity, almost completely automated, precision, high quality of representation, and, most importantly, availability of specific tools to manipulate the 3-D model. Some parameters of the flee such as the size, the surface area, and the volume were directly measured on the 3-D reconstructed object. The contour and surface are fractal with fractal dimensions of 1.4 and 2.4, respectively. Local curvatures were also calculated and show great deformations of the surface due to anisotropic growth within the floc. Discrete smooth interpolation constitutes a powerful tool to improve the understanding of the flee structure complexity.