Significance Flavin-based electron bifurcation (FBEB) is a long-hidden mode of energetic coupling in which an endergonic electron transfer process is coupled to an exergonic one. The function of the few FBEB complexes described so far is to achieve ferredoxin reduction at the negative redox limit of the biological redox window. Here, a membrane-associated FBEB complex, isolated and characterized from an anaerobic, aromatic compound-degrading bacterium, achieves a redox reaction beyond this limit possibly by two consecutive FBEB events, with reduced ferredoxin serving as donor. The benzene ring-reducing class II benzoyl-CoA reductase has a [Bam(BC) ₂ DEFGHI] ₂ composition and represents, with 4 W, 2 Se, 6 FAD, and >50 FeS cofactors, one of the most complex electron transfer machineries in nature.