Baker's yeast fermentation produces high strength wastewaters containing high concentrations of organic materials that cannot easily be degraded by biological processes. A two-stage (anaerobic-aerobic) treatment system has been used in order to treat this effluent efficiently. Most of the COD reduction takes place during the anaerobic degradation. The objective of this study was to generate a simple mathematical model for the anaerobic stage. The model, which is based on the elimination of GOD, assumes that only three consecutive reactions namely hydrolysis, acidogenesis and methanogenesis are significant. In the laboratory-scale experiments, feed strength was increased from 3600 to 14000 mg O-2 l(-1) in a batch reactor. Three reaction rate constants were found as 0.08, 0.004, 0.06 h(-1) by analyzing data from the laboratory experiments. The model was tested and found to be congruent with the daily operation data, which were collected from the Pakmaya Baker's Yeast Kocaeli Factory Plant. This plant contains three Upflow Anaerobic Sludge Blanket reactors, which are run in series. The rate constants of the hydrolysis and methanogenesis were similar to the batch experiment case. (C) 2000 Elsevier Science Ltd. All rights reserved.