The genus Clostridium as presently constituted is phylogenetically and phenotypically incoherent. Polyphasic taxonomic data indicate that the genus comprises a collection of very heterogeneous species. Numerous phylogenetic studies, principally based on sequencing of the 16S rRNA gene, indicate that the genus Clostridium should be restricted to Clostridium cluster I as Clostridium sensu stricto. Despite these findings, authors continue to add new species to the genus Clostridium that do not fall within the radiation of cluster I and the type species C. butryicum thus perpetuating the confusion associated with the taxonomy of this group. Here I formally propose that members of the Clostridium (Prazmowski) be restricted to the type species Clostridium butyricum and cluster I species. Eubacterium moniliforme, Eubacterium tarantellae, Sarcina maxima, and Sarcina ventriculi should be transferred to the genus Clostridium as Clostridium moniliforme comb. nov., Clostridium tarantellae comb. nov., Clostridium maximum comb. nov., and Clostridium ventriculi comb. nov. A novel genus Hathewaya gen. nov. is proposed for the species Clostridium histolyticum, Clostridium limosum and Clostridium proteolyticum as Hathewaya histolytica gen. nov. com. nov., Hathewaya limosa com. nov. and Hathewaya proteolytica comb. nov. The type species of Hathewaya is Hathewaya histolytica.