In nature, cellulose degradation is performed by complex microbial communities in synergistic fashion. The enrichment in mixed cultures from nature of new microbial consortia with high cellulolytic activity is essential in the identification of novel organisms, novel metabolic capabilities and novel functions that will enhance our fundamental understanding of how the potential benefits of CBP (consolidated bioprocessing) can be realized at the industrial scale. In this work we identify the key players in cellulolytic enrichment cultures with thermophilic compost as an inoculum. Community structure and functional diversity have been characterized with clone libraries targeting both the 16S rRNA gene and glycosyl hydrolase gene fragments from mixed microbial cultures. Our studies have revealed varying levels of diversity and community composition, with a narrower range of novel and very specific clostridial cellulose degraders playing the main functional role in these cultures.