The human intestinal microbiota is essential for the conversion of the dietary lignan secoisolariciresinol diglucoside (SDG) via secoisolariciresinol (SECO) to the enterolignans enterodiol (ED) and enterolactone (EL). However, knowledge of the species that catalyse the underlying reactions is scant. Therefore, we focused our attention on the identification of intestinal bacteria involved in the conversion of SDG. Strains of Bacteroides distasonis, Bacteroides fragilis, Bacteroides ovatus and Clostridium cocleatum, as well as the newly isolated strain Clostridium sp. SDG-Mt85-3Db, deglycosylated SDG. Demethylation of SECO was catalysed by strains of Butyribacterium methylotrophicum, Eubacterium callanderi, Eubacterium limosum and Peptostreptococcus productus. Dehydroxylation of SECO was catalysed by strains of Clostridium scindens and Eggerthella lenta. Finally, the newly isolated strain ED-Mt61/PYG-s6 catalysed the dehydrogenation of ED to EL. The results indicate that the activation of SDG involves phylogenetically diverse bacteria, most of which are members of the dominant human intestinal microbiota.