In Drosophila photoreceptors, phospholipase C (PLC) and other signalling components form multiprotein structures through the PDZ scaffold protein INAD. Association between PLC and INAD is important for termination of responses to light; the underlying mechanism is, however, unclear. Here we report that the maintenance of large amounts of PLC in the signalling membranes by association with INAD facilitates response termination, and show that PLC functions as a GTPase-activating protein (GAP). The inactivation of the G protein by its target, the PLC, is crucial for reliable production of single-photon responses and for the high temporal and intensity resolution of the response to light.