In this work bacteria and methanogenic archaea utilizing agricultural wastes in a pilot-scale biogas reactor were examined using sequencing and terminal restriction fragment length polymorphism analysis. Based on the analyses of 16S rRNA genes, Clostridia represented the most diverse group in the digester. Of the Clostridia, unclassified Clostridiales and the members of the genera Anaerotruncus and Tissierella were detected at high abundances. The representatives of the bacterial phyla Bacteroidetes and Proteobacteria were also defined, but in minor proportions, and were assigned to non-dominant communities. Within the phylum Euryarchaeota, the members of the orders Methanosarcinales and Methanomicrobiales were found at high levels. Methanogenic archaea were analyzed using both 16S rRNA and mcrA genes. Actually good results were received using both approaches; however, the rRNA gene method missed the non-dominant order Methanobacteriales.