Archaerhodopsin (aR), a light-driven proton pump found in HaIonubrum sp. aus-l, was crystallized into an octahedral crystal belonging to the space group P4$_3$2$_1$2. It is shown that aR is composed of7 helical segments and an anti-para1leI ${eta}$ sheet. The main-chain sIrudure of aR is nearly identical to that of bacteriorhodopsin, but a significant structural difference is observed in the protein surface, especially at lipid binding sites.