This study is devoted to the triphasic in situ rheological characterisation of an activated sludge, with total suspended solid (TSS) ranging from 10 to 35 g/L, and operated in a bioreactor under different stirring and aeration rates. The originality of this work is that flow properties are directly measured inside the bioreactor. Under low mechanical shear rates (below 0.1 s(-1)) the internal structure of the suspension (configuration of structural units) is driven by the air plume. Due to the shearing of air bubbles, apparent viscosities are strongly lowered by the injection of air but almost independent of the quantity of air (in the range 2-6 L/min). Under high mechanical shear rates (above 100 s(-1)), the configuration of structural units (i.e. floes in the case of activated sludge) is only dependent on the mechanical shearing and totally independent on the presence or absence of air. The viscosity of the broth is constant whatever the air flow rate is (0-6 L/min). We also observe a decrease in shear-thinning properties of aerated suspensions compared to non-aerated one, with a plateau above 2 L/min. The effect of TSS at constant air flow rate shows that an increase in TSS induces an increase not only in apparent viscosities but also in shear-thinning properties. (c) 2007 Elsevier B.V. All rights reserved.