The crystal structure of Orai from "Drosophila melanogaster" has been determined at 3.35 angstrom resolution (). The calcium channel is composed of a hexameric assembly of Orai subunits arranged around a central ion pore. The pore traverses the membrane and extends into the cytosol. A ring of glutamate residues on its extracellular side forms the selectivity filter. A basic region near the intracellular side can bind anions that may stabilize the closed state. The architecture of the channel differs markedly from other ion channels and provides insight into the principles of selective calcium permeation and gating.