The crystal structure of congerin II, a galectin family lectin from conger eel, was determined at 1.45A resolution. The previously determined structure of its isoform, congerin I, had revealed a fold evolution via strand swap; however, the structure of congerin II described here resembles other prototype galectins. A comparison of the two congerin genes with that of several other galectins suggests acceralated evolution of both congerin genes following gene duplication. The presence of a Mes (2-[N-morpholino]ethanesulfonic acid) molecule near the carbohydrate-binding site in the crystal structure points to the possibility of an additional binding site in congerin II. The binding site consists of a group of residues that had been replaced following gene duplication suggesting that the binding site was built under selective pressure. Congerin II may be a protein specialized for biological defense with an affinity for target carbohydrates on parasites' cell surface.