Photoactivated adenylyl cyclase (PAC) is the blue-light receptor flavoprotein recently identified as a sensor for photomovement in the unicellular flagellate, Euglena gracilis. PAC consists of α- and β-subunits, which are similar to each other and contain two FAD-binding domains (BLUF, sensors of Blue-Light Using FAD) each followed by a class III adenylyl cyclase catalytic domain. Purified PAC adenylyl cyclase activity is elevated by UV/blue-light irradiation in a fluence-dependent manner. Heterologously expressed FAD-binding domain of PACα exhibits spectral red shift upon blue-light irradiation, which indicates that the primary mechanism of photoactivation is similar to that seen with other prokaryotic BLUF proteins. Since PAC is directly activated by light without any other intervening components, it is expected to serve as a biological tool for controlling intracellular cAMP levels using light.