The 21-kDa calcium-binding domain (VI) of the small subunit of rat calpain II has been expressed in Escherichia coli, purified, and crystallized. Two orthorhombic crystal forms have been obtained: space group P212121 with a = 50.3, b = 56.5, c = 141.3 Å; and space group C2221 with a = 69.4, b = 73.9, c = 157.4 Å. Diffraction data have been collected to 2.4 Å. Sedimentation equilibrium, dynamic light scattering, and gel-permeation chromatography indicate that domain VI exists as a homodimer in solution. In accordance with the protein's behavior in solution, each crystal form contains two molecules per asymmetric unit. Screening for heavy-atom derivatives is in progress. To decrease the sensitivity to mercurials and to aid in the search for useful derivatives, Cys-to-Ser mutants have been prepared, expressed, and crystallized.