Microbial sulfate reduction with acetate as carbon source and electron donor was investigated at salinity levels between 0.53 and 1.48%. The experiment was carried out in a 2.3-1 upflow anaerobic sludge blanket reactor inoculated with granular methanogenic sludge. A pH of 8.3, a temperature of 32 +/- 1 degrees C and a chemical oxygen demand (COD)/SO4(2-)-S ratio of 2 were maintained in the reactor throughout the experiment. Sulfate reduction and the composition of the dominant bacterial communities in the reactor were monitored. The results showed that a maximal conversion rate for SO4(2-)-S of 14 g l(-1) day(-1) and a conversion efficiency of more than 90% were obtained at a salinity level of 1.26-1.39%. A further increase in the salinity level led to reactor instability. Denaturant gradient gel electrophoresis of 16S rDNA fragments amplified by PCR from total bacterial DNA extracted from the inoculum and reactor sludge showed that salinity level had an impact on the composition of the bacterial communities in the reactor. However, no clear relationship was found between reactor performance and the composition of the dominant bacterial communities in the reactor.