The role of type A Clostridium perfringens in foal necrotizing enteritis and canine acute hemorrhagic diarrhea is poorly understood. However, recently, we described a highly significant association between the presence of novel toxigenic C. perfringens and these specefic enteric diseases. These strains produce three novel putative toxins related to the beta-sheet pore-forming toxin family, which were designated NetE, NetF, and NetG. Our group demonstrated that NetF is likely the major virulence factor in strains responsible for canine acute hemorrhagic diarrhea and foal necrotizing enteritis. This thesis contains three studies aimed at expanding the understanding of netF-positive C. perfringens. In the first study, the genome of two NetF-producing strains, JFP838 and JFP55, which were isolated from cases of canine acute hemorrhagic diarrhea and foal necrotizing enteritis, respectively, were sequenced. Results of this research indicated that NetF and NetE were encoded by the same pathogenicity locus (NetF locus) on a tcp-conjugative plasmid and that NetG was encoded by a second plasmid (pNetG) on a unique pathogenicity locus (NetG locus). In addition, these two isolates shared three plasmids, including the netF/netE plasmid (pNetF), a cpe/cpb2 plasmid (pCPE), and a small bacteriocin-encoding plasmid (pBCN). In the second study, 30 NetF-producing strains were sequenced to determine whether the newly described pathogenicity loci are conserved, and all netF-positive isolates harbour the same plasmid profile. In addition, the genetic relatedness of these isolates were examined by core genome multilocus sequence typing (cgMLST). The bioinformatics analysis showed that in pNetF and pNetG, if present, pathogenicity loci are highly conserved among netF-positive strains. The common plasmid profile (pNetF, pCPE, and pBCN) was found in all NetF-producing strains and these strains were found to belong to two clonal populations. In the third study, pore-forming activity of the NetF toxin and the chemical nature of its receptor was investigated. The results demonstrated for the first time that NetF is able to create large pores on cell membranes and that its binding site is most likely a sialic acid-containing glycoprotein. Overall, this thesis has contributed significantly to the current knowledge of evolution, relationships, and pathogenesis of netF-positive C. perfringens infections.