The aim of this paper is to study the possibility to use the pertechnetate ion ((TcO4-)-Tc-99m) as a radiotracer for the measurement of the residence time distribution (RTD) of the liquid phase in opaque anaerobic digesters. An anaerobic digester in a sugar wastewater treatment plant was selected to carry out this study. Laboratory tests showed that the measured redox potential of the medium was not enough for the chemical reduction of the species TcO4- to TcO(OH)(2). However, sorption rate between 0.045 and 0.083% h(-1) were observed, probably associated to the retention of the tracer by the biomass as a result of the metabolization and catalytic reduction due to microbiological activity. This phenomenon did not affect the estimation of the RTD. The selected tracer has been used to determine the RTD of the industrial digester in a sugar wastewater treatment plant and to model the flow behavior. (C) 2002 Elsevier Science B.V. All rights reserved.