Cyclase-associated protein (CAP) is a conserved two-domain protein that helps to activate the catalytic activity of adenylyl cyclase in the cyclase-bound state through interaction with Ras, which binds to the cyclase in a different region. With its other domain, CAP can bind monomeric actin and therefore also carries a cytoskeletal function. The protein is thus involved in Ras/cAMP-dependent signal transduction and most likely serves as an adapter protein translocating the adenylyl cyclase complex to the actin cytoskeleton. Crystals belonging to the orthorhombic space group C222, with unitcell parameters a = 71.2, b = 75.1, c = 162.9 A Ê , have been obtained from Dictyostelium discoideum CAP carrying a C-terminal His tag. A complete native data set extending to 2.2 A Ê resolution was collected from a single crystal using an in-house X-ray system. The asymmetric unit contains one molecule of CAP.