The Clostridia are a highly polyphyletic class of Firmicutes, including Clostridium and other similar genera. They are distinguished from the Bacilli by lacking aerobic respiration. They are obligate anaerobes and oxygen is toxic to them. Species of the genus Clostridium are all Gram-positive and have the ability to form spores. Studies show they are not a monophyletic group, and their relationships are not entirely certain. Currently most are placed in a single order called Clostridiales, but this is not a natural group and is likely to be redefined in the future.
Most species of the genus Clostridium are saprophytic organisms found in many places in the environment, most notably the soil. However, the genus does contain some human pathogens (outlined below). The toxins produced by certain members of the Clostridium genus are among the most dangerous known. Examples are tetanus toxin (known as tetanospasmin) produced by C. tetani and botulinum toxin produced by C. botulinum.
Notable species of this class include:
- Clostridium perfringens (Gangrene, Food poisoning)
- Clostridium difficile (Pseudomembranous colitis)
- Clostridium tetani (Tetanus)
- Clostridium botulinum (Botulism)
- Clostridium acetobutylicum
- Clostridium haemolyticum
- Clostridium novyi
- Clostridium oedematiens
Heliobacteria are also members of the class Clostridia.
Some of the enzymes produced by this group are used in bioremediation.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)  and National Center for Biotechnology Information (NCBI) and the phylogeny is based on 16S rRNA-based LTP release 106 by The All-Species Living Tree Project 
Unassigned Clostridia s.s.
- Catabacteraceae ♠ Lau et al. 2007
- Catabacter hongkongensis ♠ Lau et al. 2007
- Christensenellaceae Morotomi et al. 2012
- Christensenella minuta Morotomi et al. 2012
- Defluviitaleaceae Jabari et al. 2012
- Defluviitalea saccharophila Jabari et al. 2012
♠ Strains found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature (LSPN)
- J.P. Euzéby. "Clostridia". List of Prokaryotic names with Standing in Nomenclature (LPSN) . Retrieved 2011-11-17.
- Sayers et al. "Clostridia". National Center for Biotechnology Information (NCBI) taxonomy database . Retrieved 2011-06-05.
- All-Species Living Tree Project."16S rRNA-based LTP release 106 (full tree)". Silva Comprehensive Ribosomal RNA Database . Retrieved 2011-11-17.