We used two new coral fluorescent proteins as fluorescence resonance energy transfer (FRET) donor and acceptor to develop a voltage sensor, named Mermaid, that displays approximately 40% changes in emission ratio per 100 mV, allowing for direct visualization of electrical activities in cultured excitable cells. Notably, Mermaid has fast on-off kinetics at warm (approximately 33 degrees C) temperatures and can report voltage spikes comparable to action potentials.