The competition between acetate utilising sulphate reducing bacteria (ASRB) and methane producing bacteria (AMB) was studied in an upflow anaerobic sludge bed (UASB) reactor (superficial liquid upflow velocity of 1 m/h) treating an acetate and sulphate mixture (COD:sulphate ratio 0.5) at 30 degrees C and pH 8. The competition was characterised in terms of reactor performance (GOD balance) and by microbiological analysis (16S rRNA profile) of the biomass. The following measures of steering the competition between ASRB and AMB were studied: (a) the addition of a pure culture of an acetotrophic SRB (Desulforhabdus amnigenus) to the reactor; and (b) the use of shock treatments (pH drop or air exposure). Addition of D. amnigenus was not successful, even after operating the reactor under batch conditions for 14 days. The change Of pH from 8 to 7 caused an increase of the free hydrogen sulphide (FS) concentration in the reactor from 5-10 to 90-125 mg S-FS/l. This exerted a small effect on the competition between ASRB and AMB, and the amount of acetate consumed by SRB increased around 5%. A short-term air exposure (24 h) strongly deteriorated the reactor performance and about 20 days were required to recover to previous conditions. However, ASRB were able to outcompete AMB in the subsequent days of operation. The 16S rRNA characterisation of the biomass showed that Desulfobacterium or relatives and Desulfotomaculum acetoxidans-like bacteria were responsible for the sulphidogenic degradation of acetate in this reactor: (C) 1997 Elsevier Science Ltd.