Measurable changes occurred in the electrical impedance of growing cultures ofDesuljovibrio aestuariias compared with sterile media. The ratio of these changes (electrical impedance ratio,RI) was measured continuously using a Bactometer 32, a magnetic tape recorder, and a computer for data retrieval. The rate of sulfate reduction correlated with the rate of change inRIinD. aestuariicultures and sediment slurries from San Francisco Bay. Both sulfide and iron were required for maximalRIresponse. Molybdate (5–20 mM) totally inhibited both sulfate reduction and changes inRIinD. aestuariicultures. Molybdate (20 mM) completely inhibited sulfate reduction in sediment slurries as measured by the impedance ratio technique or35SO42‐reduction. Sulfate reduction in sediment slurries was stimulated by lactate but not by formate, acetate, or pyruvate. The rates of sulfate reduction in San Francisco Bay sediment, estimated in vitro by the impedance ratio technique, ranges from 0.85 to 1.78 mmol/liter/lbday and compared favorably with estimates made by33SO42‐reduction.