Clostridium perfringens causes human gas gangrene and food poisoning as well as several enterotoxemic diseases of animals. The organism is characterized by its ability to produce numerous extracellular toxins including alpha-toxin or phospholipase C, theta-toxin or perfringolysin O, kappa-toxin or collagenase, as well as a sporulation-associated enterotoxin. Although the genes encoding the alpha-toxin and theta-toxin are located on the chromosome, the genes encoding many of the other extracellular toxins are located on large plasmids. The enterotoxin gene can be either chromosomal or plasmid determined. Several of these toxin genes are associated with insertion sequences. The production of many of the extracellular toxins is regulated at the transcriptional level by the products of the virR and virS genes, which together comprise a two-component signal transduction system.