Strictly anaerobic spp. are characterized by their vast metabolic potential. As members of the Prevotellaceae family, they represent the most abundant organisms in the rumen and are typically found in monogastrics such as pigs and humans. Within their largely anoxic habitats, these bacteria are considered to rely primarily on fermentation for energy conservation. A recent study of the rumen microbiome identified multiple subunits of the Na -translocating NADH:quinone oxidoreductase (NQR) belonging to different spp. Commonly, the NQR is associated with biochemical energy generation by respiration. The existence of this Na pump in spp. may indicate an important role for electrochemical Na gradients in their anaerobic metabolism. However, detailed information about the potential activity of the NQR in spp. is not available. Here, the presence of a functioning NQR in the strictly anaerobic model organism B 4 was verified by conducting mass spectrometric, biochemical, and kinetic experiments. Our findings propose that B 4 and other spp. retrieved from the rumen operate a respiratory NQR together with a fumarate reductase which suggests that these ruminal bacteria utilize a sodium motive force generated during respiratory NADH:fumarate oxidoreduction.