The paper presents rheological test results of activated sludge from municipal-industrial treatment plants. The tested sludge behaved like a plastic-viscous fluid. The best matching was achieved for the Casson, Ostwald-de Waele and Herschel-Bulkley models. Correlation of rheological parameters, i.e. fluid consistency index (K), yield stress (tau(0)), flow behaviour index (n) and plastic viscosity (eta(P)) is also presented.