The rheological characterisation of activated sludge originating from different aeration tanks (ATs) and from lab scale plants was carried out using rotation tests. Two rheological parameters from Bingham's model were used: Viscosity (mu(B)) and shear stress (tau(B)). Viscosity and shear stress were strongly influenced by the total suspended solid (TSS) content of sludge, an exponential relationship being found between TSS and the Bingham's rheological parameters. Significant differences in sludge viscosity and shear stress were found depending on their sampling origin, indicating the ability of these parameters to show different sludge qualities. During aerobic and anaerobic digestion of sludge, important variations in shear stress were noticed, according to the digestion duration of sludge. The evolution in Bingham's viscosity was influenced by the viscosity of the sludges' supernatant. The sludges were also modified by ageing at low concentrations of O-2 or chlorination. These modifications produced a decrease in shear stress that can be interpreted as a decrease in floc strength. Bingham's parameters seem to be able to detect the evolution in sludges quality and in floc strength even at low total suspended solid as the one's usually found in activated sludge processes (less than 10 g l(-1)). (C) 2003 Elsevier Ltd. All rights reserved.