1. Calculation showed that rates of sulphate reduction by the Hildenborough strain of Desulfovibrio vulgaris in non-nutrient buffer with gaseous hydrogen as electron donor were as high as, or higher than, the maximum possible rate of sulphate reduction during growth. 2. Respirometry showed that H2 oxidation was completely suppressed during growth unless the whole electron transport system was bypassed with a redox dye or the sulphate reductase system (terminal electron acceptor) was bypassed with FMN. 3. Growth experiments with intermittent sulphate feeds showed that cells under H2 “wasted” most of the sulphate by a reaction unconnected with growth, and thus made much less growth than did similar cultures under N2. 4. H2 exerted no discernible “lactate-sparing” action (that is, it did not serve as an alternative energy source) in low-lactate medium.