The 16S rRNA sequences of 17 species of the genus Clostridium were determined by direct sequencing of their PCR amplified genes. The sequences were aligned with those from other known clostridial species and representative low G + C Gram-positive relatives, and a phylogenetic tree was constructed. It was evident from the comparative sequence analysis that the genus Clostridium as presently constituted is phylogenetically extremely heterogeneous. This study corroborates and extends earlier findings in showing that many non-sporeforming bacteria are phylogenetically closely intermixed with Clostridium species. The taxonomic implications of the phylogenetic findings are discussed.