Cadherins are calcium-dependent cell adhesion molecules containing extracellular repeats of approximately 110 amino acids. The three-dimensional structure of the amino-terminal repeat of mouse epithelial cadherin was determined by multidimensional heteronuclear magnetic resonance spectroscopy. The calcium ion was bound by a short alpha helix and by loops at one end of the seven-stranded beta-barrel structure. An exposed concave face is in a position to provide homophilic binding specificity and was also sensitive to calcium ligation. Unexpected structural similarities with the immunoglobulin fold suggest an evolutionary relation between calcium-dependent and calcium-independent cell adhesion molecules.