AbstractRelatively large activated sludge floes (larger than about 100 μm) were stabilized, using a histological tissue specimen preparation procedure, and then were sliced into sections of 3 to 6 μm thick. The study of these sections, after staining, revealed the internal structure of the activated sludge floes. No uniformity of this structure was found. The distribution of microorganisms and of extracellular polymers (EPs) in the floes varied randomly on the plane of the sections and along the dimension perpendicular to the plane, leaving large water channels and reservoirs in some of the floes. The lack of a characteristic size for the water gaps in the floes and a general self‐similar appearance of the sections suggested that the activated sludge floes might be characterized by the fractal concept within a certain size limit. Direct observation of the interior of the floes indicated an abundant presence of extracellular polymers (EPs) in amorphous forms, surrounding microorganisms in most of the fl