Extrinsic absorption changes exhibited by squid giant axons stained with the voltage-sensitive merocyanine-oxazolone dye NK 2367 were measured during brief voltage-clamp steps. Experiments that employed an optical recording system having a sub-microsecond response time constant demonstrate that this dye responds to step changes in membrane voltage in less than 2 microseconds at room temperature. The optical response is independent of ionic currents and is unlikely to depend upon cytoskeletal or cytoplasmic events.