Endophytic clostridia present on various plants as obligate anaerobes were surveyed by terminal restriction fragment length polymorphism (TRFLP) analysis specific to the clostridial 16S rRNA gene. Endophytic clostridia were detected in 10 plant types: sugarcane, cultivated rice, corn, tobacco, soybean, bermuda grass, tall fescue, and three mangrove species. Phylogenetically, cluster XIVa clostridia were detected more frequently than cluster I clostridia in aerial parts. Isolation of clostridia from surface-sterilized sugarcane stem validated the TRFLP results. Plant-derived clostridia occupied two unique phylogenetic positions (groups I and II) within cluster XIVa. Most of cluster XIVa clostridia from other sources (e.g., human, animal, and insect intestines) were located outside these groups. Thus two unique groups of cluster XIVa clostridia are widely distributed in plants, including crops. In field-grown soybeans, TRFLP analysis revealed clostridia only in a non-nodulating mutant. Ribosomal intergenic spacer analysis (RISA) showed that the bacterial community in soybean shoot depended partly on the soybean nodulation genotype.