Engineering microbial consortia capable of efficient ethanolic fermentation of cellulose is a strategy for the development of consolidated bioprocessing for bioethanol production. Co-cultures of cellulolytic Clostridium thermocellum with non-cellulolytic Thermoanaerobacter strains (X514 and 39E) significantly improved ethanol production by 194-440%. Strain X514 enhanced ethanolic fermentation much more effectively than strain 39E in co-cultivation, with ethanol production in X514 co-cultures at least 62% higher than that of 39E co-cultures. Comparative genome sequence analysis revealed that the higher ethanolic fermentation efficiency in strain X514 was associated with the presence of a complete vitamin B<SUB>12</SUB> biosynthesis pathway, which is incomplete in strain 39E. The significance of the vitamin B<SUB>12</SUB>de novo biosynthesis capacity was further supported by the observation of improved ethanol production in strain 39E by 203% following the addition of exogenous vitamin B<SUB>12</SUB>. The vitamin B<SUB>12</SUB> biosynthesis pathway provides a valuable biomarker for selecting metabolically robust strains for bioethanol production.