The objective of this study was to evaluate the effect of extracellular H-2 on organic acid utilization by two lactate-utilizing strains of Selenomonas ruminantium (HD4, H18). Both strains were able to grow (optical density at 600 nm greater than or equal to after 9 h) on either aspartate, fumarate, or malate in the presence of 1 atmosphere (atm) of H-2. Succinate was the major end product produced in these fermentations. When cells were incubated with lactate plus 1 atm H-2, growth was minimal and little lactate was fermented. The electron transport inhibitor, acriflavine, was a strong inhibitor of growth when either strain was incubated in the presence of organic acid plus H-2. Compared with glucose- or lactate-grown cells, cellular carbohydrate levels were lower for both strains in cells grown on either organic acid plus H-2. These results suggest that electron transport plays a role in organic acid utilization by S. ruminantium.