Sulfate is present in all freshwater and marine environments and is reduced to toxic and corrosive hydrogen sulfide by anaerobic bacteria. By measuring depth profiles of sulfate in sediments, it is possible to obtain estimates of sulfate reduction rates (SRRs). A whole-cell microscale biosensor for sulfate was constructed by placing a mixture of sulfate-reducing and aerobic bacteria in front of a hydrogen sulfide microsensor. The bacteria were supplied with electron donors from a reservoir behind the bacterial biomass. The aerobic bacteria ensured anaerobic conditions in deeper parts of the 150-mu m-thick bacterial layer, so that the sulfate reducers could be active. A typical tip diameter of a sulfate biosensor is 50 mu m. The calibration curve was close to linear in the 0-2 mM range, with a detection limit of about 10 mu M. The 90% response time varied between 90 and 220 s, slowest response at low concentrations. Oxygen and ferrous iron at environmental concentrations gave no interference, but there was a 6% signal difference between stirred and stagnant liquid medium. The sensitivity of the biosensor could be modulated by applying a charge between sensor interior and an external electrode. The lifetime is usually a few weeks. The biosensor was used to measure sulfate profiles in a freshwater sediment.