Fluorescent protein voltage sensors for measuring membrane potential and imaging high-frequency neuronal electrical activity are disclosed. In particular, the invention relates to engineered protein voltage sensors that comprise a voltage-sensing domain comprising four transmembrane domains linked to a circularly permuted fluorescent protein, which is inserted into the extracellular loop between the third (S3) and fourth (S4) transmembrane segments of the voltage-sensing domain. Such fluorescent protein voltage sensors can be used for measuring the electrical activity of neurons, including single action potentials, trains of action potentials, and subthreshold potential changes and, in particular, for imaging high-frequency neuronal electrical activity. Additionally, fluorescent protein voltage sensors can be used for a variety of other purposes, including measuring the membrane potential of any cell, including other excitable cells such as cardiac cells and endocrine cells, and for screening agents that target ion channels for their effects on membrane potential.