Researchers have long sought a method for non-invasively measuring changes in membrane potential; however, existing genetically encoded voltage indicators (GEVIs) generate weak signals that cannot be detected in intact tissues. The authors engineered a new GEVI, ArcLight, with an improved signal-to-noise ratio that enabled them to record from individual neurons in the intact Drosophila melanogaster brain. ArcLight can signal both subthreshold changes in membrane potential and action potentials, opening the door to further studies of electrical activity in intact neural circuits.