The gene sequences encoding the 16S rRNA of Clostridium felsineum DSM 794T and NCIMB 10690T were determined. Both sequences exhibited a relatively very low degree of similarity to the previously determined 16S rRNA gene sequence from C. felsineum DSM 794T. C. felsineum is a member of the major Clostridium cluster, cluster I, and is phylogenetically closely related to Clostridium acetobutylicum. DNA-DNA hybridization results clearly indicated that C. felsineum and C. acetobutylicum belong to distinct species.