This study investigated the effect of feed sulphate concentration on the kinetics of anaerobic sulphate reduction by a mixed SRB culture, using lactate as the sole carbon source and electron donor. Chemostat cultures were operated across a range of residence times (0.55 d) and feed sulphate concentrations (1.010.0 g l(-1)). Similar phenomena were observed at feed sulphate concentrations of 1.0 and 10.0 g l(-1) with the volumetric sulphate reduction rate increasing linearly with increasing volumetric sulphate loading rate. These reactors were characterised by higher specific volumetric sulphate reduction rates with maximum values of 0.24 and 0.20 g h(-1) g(-1). Contrastingly, the reactors fed with sulphate concentrations of 2.5 and 5.0 g l(-1) showed distinctly different trends in which the volumetric sulphate reduction rate passed through a maximum at the dilution rates of 0.014 and 0.021 h(-1), respectively, followed by a decline with further increase in sulphate loading rate. The maximum specific volumetric sulphate reduction rates observed were 26-fold lower than those observed at 1.0 and 10.0 g l(-1) feed sulphate concentrations. Profiles of specific volumetric sulphate reduction rate and biomass concentration suggested a shift in lactate utilisation from oxidation to fermentation at high dilution rates, implying a change in the dominant components of the microbial consortium. The data suggest that population structure was influenced by lactate affinity and dissolved sulphide concentration. The trends observed were attributed to the greater ability of lactate oxidisers to scavenge lactate under limiting concentrations of the substrate and their greater resilience to dissolved sulphide species in comparison to lactate fermenters. (C) 2010 Elsevier Ltd. All rights reserved.