Anaerobic nitrogen-fixing consortia consisting of N2-fixing clostridia and diverse nondiazotrophic bacteria were previously isolated from various gramineous plants (K. Minamisawa, K. Nishioka, T. Miyaki, B. Ye, T. Miyamoto, M. You, A. Saito, M. Saito, W. Barraquio, N. Teaumroong, T. Sein, and T. Tadashi, Appl. Environ. Microbiol. 70:3096-3102, 2004). For this work, clostridial populations and their phylogenetic structures in a stand of the grass Miscanthus sinensis in Japan were assessed by a 16S rRNA gene-targeted terminal restriction fragment length polymorphism (TRFLP) analysis combined with most-probable-number (MPN) counts. PCR primers and restriction enzymes were optimized for analyses of the plant clostridia. Clostridia were detected in strongly surface-sterilized leaves, stems, and roots of the plants at approximately 104 to 105 cells/g of fresh weight; they made up a large proportion of N2-fixing bacterial populations, as determined by MPN counts associated with an acetylene reduction assay. Phylogenetic grouping by MPN-TRFLP analysis revealed that the clostridial populations belonged to group II of cluster XIVa and groups IV and V of cluster I; this result was supported by a culture-independent TRFLP analysis using direct DNA extraction from plants. When phylogenetic populations from M. sinensis and the soil around the plants were compared, group II clostridia were found to exist exclusively in M. sinensis.