During the anaerobic digestion process, a significant part of the organic matter in sewage sludge is decomposed to form other organic and inorganic compounds in dissolved form. This biological transformation of a substantial part of the organic solids has, certainly, a strong influence on the rheological characteristics of the sludge. In this paper a test facility was set up to simulate sewage sludge digestion and periodic observations on the evolution of the sludge characteristics were carried out. Results of this study show that important changes on the sludge rheological behaviour occur during anaerobic digestion and that the evolution of those changes is related to the degree of digestion. Moreover, it is shown that the verified high degree of physical changes can not be explained only by the total solids concentration variation and two hypotheses are proposed to explain those changes.