HETEROTRIMERIC G proteins couple various receptors to intracellular effector molecules. Although the role of the G alpha subunit in effector activation, guanine nucleotide exchange and GTP hydrolysis has been well studied(1-4), the cellular functions of the G beta subunits are less well understood(5,6). G beta gamma dimers bind G alpha subunits and anchor them to the membrane for presentation to the receptor(7,9). Tn specific systems, the G beta subunits have also been implicated in direct coupling to ion channels and to effector molecules(10-19). We have isolated Drosophila melanogaster mutants defective in an eye-specific G-protein beta-subunit (G beta e), and show here that the beta-subunit is essential for G-protein-receptor coupling in vivo. Remarkably, G beta mutants are also severely defective in the deactivation of the light response, demonstrating an essential role for the G beta subunit in terminating the active state of this signalling cascade.