infection (CDI) is a major cause of hospital-associated， antibiotic-induced diarrhea， which is largely mediated by the production of two large multidomain clostridial toxins， TcdA and TcdB. Both toxins coordinate the action of specific domains to bind receptors， enter cells， and deliver a catalytic fragment into the cytosol. This results in GTPase inactivation， actin disassembly， and cytotoxicity. TcdB in particular has been shown to encode a region covering amino acids 1753 to 1851 that affects epitope exposure and cytotoxicity. Surprisingly， studies here show that several peptides derived from this region， which share the consensus sequence NVFKGNTISDK， protect cells from the action of TcdB. One peptide， PepB2， forms multiple interactions with the carboxy-terminal region of TcdB， destabilizes TcdB structure， and disrupts cell binding. We further show that these effects require PepB2 to form a higher-order polymeric complex， a process that requires the central GN amino acid pair. These data suggest that TcdB interacts with repeat sequences in the proximal carboxy-terminal domain of TcdB (i.e.， the CROP domain) to alter the conformation of TcdB. Furthermore， these studies provide insights into TcdB structure and functions that can be exploited to inactivate this critical virulence factor and ameliorate the course of CDI. is a leading cause of hospital-associated illness that is often associated with antibiotic treatment. To cause disease， secretes toxins， including TcdB， which is a multidomain intracellular bacterial toxin that undergoes conformational changes during cellular intoxication. This study describes the development of peptide-based inhibitors that target a region of TcdB thought to be critical for structural integrity of the toxin. The results show that peptides derived from a structurally important region of TcdB can be used to destabilize the toxin and prevent cellular intoxication. Importantly， this work provides a novel means of toxin inhibition that could in the future develop into a treatment.