Knowledge on rheological properties of sludge is important for the design of pumping and transport facilities and as a tool in process control during treatment. Concentrated activated sludge is known to behave as a non-Newtonian material. The contribution to non-Newtonian behaviour is believed to originate from the colloidal properties of solids more than from the molecular properties of the suspension. Considering that the colloidal proper-ties of sludge depend on factors like pH, conductivity. solids concentration and flocculation properties. this study examines the effects of these variables on the theology of activated sludge. Results show that the rheograms obtained fa best to a power law model indicating that the flow of activated sludge can best be expressed by the pseudoplastic flow model. It is also observed that the apparent viscosity increases sharply with the increase of solids concentration. From the pH values studied, the one that causes the lowest viscosity is the pH 5.5. As the pH increases, the viscosity measured increases considerably. This makes sense considering that the isoelectric point of bacteria is at pH 2-4. Conductivity is also found to affect the viscosity greatly; such that a the conductivity increases, the apparent viscosity decreases. Finally, experimental results reveal that the presence of extracellular polymers and the flocculated nature of activated sludge affect its viscosity.