Genetically encoded voltage indicators (GEVIs) are a promising technology for fluorescence readout of millisecond-scale neuronal
dynamics. Prior GEVIs had insufficient signaling speed and dynamic range to resolve action potentials in live animals. We
coupled fast voltage-sensing domains from a rhodopsin protein to bright fluorophores via resonance energy transfer. The resulting
GEVIs are sufficiently bright and fast to report neuronal action potentials and membrane voltage dynamics in awake mice and
flies, resolving fast spike trains with 0.2-millisecond timing precision at spike detection error rates orders of magnitude
better than prior GEVIs. In vivo imaging revealed sensory-evoked responses, including somatic spiking, dendritic dynamics,
and intracellular voltage propagation. These results empower in vivo optical studies of neuronal electrophysiology and coding
and motivate further advancements in high-speed microscopy.