Voltage sensors for in vivo applications Imaging voltage changes in single neurons in vivo has so far required averaging over multiple trials. Gong et al. now report a genetically encoded voltage indicator (GEVI) that is bright enough to enable single-trial imaging of voltage changes in living mice and flies. This GEVI consists of a rhodopsin from Acetabularia acetabulum (Ace) and the fluorescent protein mNeon-Green, which together act as a fluorescence resonance energy transfer (FRET) pair. With sixfold faster kinetics and a 50% increase in brightness compared to existing GEVIs of similar design, Ace-mNeon sensors support high-fidelity imaging of subthreshold voltage changes, single action potentials and fast spike trains in vivo. The researchers demonstrated these capabilities in the visual cortex of both anesthetized and awake mice, as well as in the Drosophila olfactory system. Gong, Y. et al. Science doi:10.1126/science.aab0810 (19 November 2015).