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Clostridium difficile TcdA is a large toxin that binds carbohydrates on intestinal epithelial cells. A 2-Å resolution cocrystal structure reveals two molecules of α-Gal-(1,3)-β-Gal-(1,4)-β-GlcNAcO(CH₂)₈CO₂CH₃ binding in an extended conformation to TcdA. Residues forming key contacts with the trisaccharides are conserved in all seven putative binding sites in TcdA, suggesting a mode of multivalent binding that may be exploited for the rational design of novel therapeutics.