Our paper deals with the description of the rheological properties of activated sewage sludge before and after disintegration. There is an assumption that disintegrated activated sewage sludge is able to change rheological properties. These changes probably cause the modification of the structure of activated sewage sludge after disintegration. The reason is that during disintegration, cell walls of organisms (from which the activated sewage sludge is composed) are disturbed. Currently there are not published many papers that deal with rheological properties of disintegrated activated sewage sludge. There is therefore no opportunity to confront our assumption with papers of other authors. The sample of activated sewage sludge was collected from aeration tank of the wastewater treatment plant for 10,000 population equivalent (PE). In our work the following rheological properties were described: temperature dependence on viscosity and the shear stress dependence on shear rate. On the basis of measured data Arrhenius mathematical model has been applied. By using this mathematical model the activation energy has been obtained.