The activated sludge process is the most widely used biological wastewater treatment method. The measurement of some physicochemical parameters in aeration tanks does not allow clarification operation failures to be avoided. This study focuses on the determination of a rheological parameter of filamentous activated sludge at standard concentrations (2-7 g L-1) in order to assess sludge settleability. On filamentous sludges, rheological measurements in shear flow display a hysteresis area (or reduced hysteresis area (rHa) when the volume of sludge considered is 1 mL), which corresponds to the presence of filamentous bacteria in the sludges. In numerous cases of filamentous activated sludges, significant relationships with the zone settlement velocity (ZSV) parameters of settleability point out that hysteresis area could be used to predict the ability of filamentous activated sludges to settle. With the protocol used, a filamentous activated sludge with a rHa > 20 Pa s(-1) mL(-1), always showed a very low or null ability to settle whatever the value of activated sludge total suspended solid (TSS) content over a standard range (2-7 g L-1) (c) 2005 Elsevier Ltd. All rights reserved.