Abstract In humans, plant cell wall polysaccharides (mainly cellulose and hemicelluloses) represent an important source of dietary fibres that are digested by the gut microflora. However, the fibrolytic micro-organisms involved in the breakdown of these substrates remain largely unknown. Our objective was to quantify the microcrystalline-cellulose-degrading and methanogenic microbial communities in faecal samples (n=34) from both methane- and non-methane-excreting individuals and to identify the predominant cellulolytic organisms in these two categories of subjects. Microcrystalline cellulose degraders could only be enumerated in faecal samples from methane excretors while this community remained undetectable in non-methane-excretors. The cellulolytic isolates corresponded to new Ruminococcus species and to Enterococcus sp. closely related to Enterococcus faecalis. The presence of such fibrolytic species seems to be linked to that of methanogenic archaea in the gut, the relationships between these two microbial communities needing further investigation. Our findings suggest that the structure and activity of the cellulolytic communities differ in methane- and non-methane-excreting individuals.