Despite the fact that complex saccharides play an important role in many biological recognition processes, molecular level descriptions of protein-carbohydrate interactions are sparse. The legume lectin concanavalin A (con A), from Canavalia ensiformis, specifically recognizes the trimannoside core of many complex glycans. We have determined the crystal structure of a con A-trimannoside complex at 2.3-A resolution now describe the trimannoside interaction with conA. All three sugar residues are in well defined difference electron density. The 1,6-linked mannose residue is bound at the previously reported monosaccharide binding site; the other two sugars bind in an extended cleft formed by residues Tyr-12, Pro-13, Asn-14, Thr-15, and Asp-16. Hydrogen bonds are formed between the protein and all three sugar residues. In particular, the 1,3-linked mannose residue makes a strong hydrogen bond with the main chain of the protein. In addition, a water molecule, which is conserved in other con A structures, plays an important role in anchoring the reducing sugar unit to the protein. The complex is further stabilized by van der Waals interactions. The structure provides a rationale for the high affinity of con A for N-linked glycans.