The bacterial population structure of sludge present in a full-scale upflow, fully-packed anaerobic digester treating a sulphate-containing wastewater (12 g COD/l, 4 g SO42-/l) from the citric acid production industry for a period of 5 years was determined: Populations of sulphate-reducing, methanogenic, syntrophic and homoacetogenic bacteria were found to prevail in the reactor sludge. The degradation patterns of several key anaerobic intermediates were studied. Both propionate and H-2/CO2 were completely metabolised by sulphate reducing bacteria. Methanogenic bacteria outcompeted sulphate-reducers for acetate, whereas competition took place for butyrate and ethanol. The outcome of competition in the sludge was not affected by the location within the reactor or by the cell residence time. (C) 1998 Elsevier Science Ltd. All rights reserved.