-Four sheep were fed an alfalfa hay diet. Rumen content samples were collected three hours after feeding in order to total microorganism population (TP), solid attached population (SAP) and solid attached firmly population (SAFP). Fibrolytic specific activities (xylanase, CMCase and β-glycosidases) were estimated by the amount of reducing sugars or p-nitrophenol released from the appropriate substrate. The distribution of the three main cellulolytic bacterial species (Fibrobacter succinogenes, Ruminococcus albus and Ruminococcus flavefaciens) was quantified by dot-blot hybridisation using specific 16S-rRNA-targeting probes. Specific activities of polysaccharidase enzymes were higher in SAP than in TP, and in SAFP than in SAP. The sum of RNA of the three cellulolytic bacterial species represented on average 9% of the total bacterial RNA, and increased after filtration. In all samples, the relative population size of F. succinogenes was higher than that of R. albus and of R. flavefaciens. These results demonstrate that the most active enzymes are secreted by the particle-associated microorganisms. The differences in composition of the microflora between the solid and liquid phase suggest that bacteria are not equally distributed throughout the rumen content: the cellulolytic species are present in a higher proportion in the solid phase of rumen contents.rumen content / fibrolytic activity / cellulolytic bacteria / oligonucleotide probe Résumé -Répartition de l'activité fibrolytique microbienne et des bactéries cellulolytiques entre phases solides et liquides du contenu ruminal. Quatre moutons, munis d'une canule du rumen, ont reçu un régime à base de foin. Des échantillons de contenus ruminaux étaient prélevés 3 h après la distribution du repas afin d'isoler la population totale de micro-organismes (TP), la population de microorganismes associés à la phase solide du contenu (SAP) et celle fermement associée à cette phase solide (SAFP). Les activités fibrolytiques spécifiques (xylanase, CMCase et β-glycosidases) étaient estimées Reprod. Nutr. Dev. 41 (2001) 187-194 187