The kinetics of sulfate reduction and cell densities of sulfate-reducing bacteria (SRB) were determined in activated sludge at Aalborg East wastewater treatment plant, a modern 100 000 person equivalent plant, where SRB are subjected to alternating cycles of oxic and anoxic conditions. The number of SRB was relatively constant over the year, ranging from 2.1x10(5) to 1.1x10(6) cells ml(-1) as determined by (35)S-radiotracer most probable number (MPN) in a growth medium prepared from anaerobic, sterilized sludge. Under anoxic conditions, the sulfide production in the activated sludge followed a biphasic pattern, being linear for approximately 5 h, followed by an exponential phase with doubling times of sulfide production of 4.2-12.6 h. Sulfate reduction started immediately after the onset of the anoxic cycle. Addition of antibiotics (chloramphenicol and streptomycin) to the activated sludge prevented the exponential phase of sulfate reduction for up to 100 h and this treatment was found to yield precise estimates of potential sulfate reduction rates. The addition of sulfate, sodium dithionite or single carbon compounds (lactate, acetate and glucose) did not decrease the length of the linear phase of sulfate reduction, nor did it affect the sulfate reduction rate. Our results indicate a tight and efficient metabolic coupling between populations of SRB and fermenting bacteria in activated sludge and a high capacity for sulfate reduction in sludge stored in settling tanks. Bacterial sulfate reduction may therefore be an important process in the destabilization of the floc structure when activated sludge is stored anaerobically for several days prior to dewatering.