Rumen strains of the obligate anaerobe Bacteroides melaninogenicus normally require medium supplemented with both heme and vitamin K. Sodium succinate was found to be an additional growth factor in that this compound can replace the requirement for heme in the presence of vitamin K, allowing good growth of the organism, and succinate can also partially replace the requirement for vitamin K in the presence of heme. The addition of succinate to a medium supplemented with both vitamin K and heme increases the growth rate of the culture. This ability to stimulate growth was specific for succinate, and cells grown without heme but with vitamin K and succinate were insensitive to cyanide. These experiments demonstrate a central role for succinate in the metabolism of B. melaninogenicus.