Rates of bacterial sulfate reduction are calculated from the accumulation of reduced sulfur compounds in coastal sediments. The method is found to underestimate the in situ metabolism 10‐fold because it neglects diffusional losses of produced sulfide. In relation to this result, the quantitative connection between the pyrite, HCl‐extractable iron, and organic carbon contents of the sediments and the intensity of sulfate reduction are disussed. A comparison is made between colony counts of sulfate‐reducing bacteria and the rate of sulfate reduction in coastal sediments. The number of bacteria is roughly proportional to their measured rate of metabolism both when different sediment types and different depths are compared. The colony counts, however, seem to underestimate the true numbers of sulfate reducers by 1000‐fold or more.