Clostridium sordellii is a bacterial pathogen of both humans and animals, associated with severe disease and high mortality rates. While the pathogenesis of C. sordellii is poorly understood, a major virulence factor of C. sordellii is the production of the toxin TcsL, also referred to as Lethal Toxin. TcsL is encoded within a region known as the Pathogenicity Locus (PaLoc). We have sequenced the genomes of 44 C. sordellii isolates of animal and human origin and fully closed the genome of C. sordellii type strain ATCC9714. In the ATCC9714 genome, the PaLoc is carried on a ~103 kb plasmid, named pCS1-1. The remaining toxigenic and 2 non-toxigenic isolates of C. sordellii contained related plasmids, designated the pCS1 family of plasmids. An ~17.3 kb region from ATCC9714 pCS1-1 encodes putative components of a Gram positive conjugation system. We have demonstrated the inter-strain conjugative transfer of pCS1-1 through the use of a derivative encoding erythromycin resistance. Insertional inactivation within the putative pCS1-1 conjugation locus was performed and phenotypic analysis of these mutants indicates that genes within this region play a significant role in the conjugative transfer of pCS1-1. Homologous conjugation loci are also present on plasmids from Clostridium botulinum and Clostridium perfringens. These results suggest that this region represents a novel clostridal conjugation locus and provides new information on the epidemiology of C. sordellii, in particular with respect to the carriage and horizontal transfer of elements encoding known virulence factors. This study also provides evidence of the evolution of, and recombination between, toxin encoding plasmids among different species of the clostridia.