Modulation of light at 3 and 6 kMc is achieved by applying a superimposed electrostatic and microwave field to a carbon-disulfide Kerr-cell which is incorporated within the high-electric-field region of a resonant cavity. The development of this light shutter requires the analysis of the Kerr effect under circumstances in which the transit time of light is appreciable. A Kerr cell whose length is such that the transit time of light is one-half the period of the modulating microwave field proves to have particular advantages over other designs. The light shutter is realized with a re-entrant microwave cavity with provision for the application of electrostatic as well as microwave fields. At about 26-kv dc and 10-kw pulsed 3-kMc ac power, the system modulates a light beam of several milliwatts radiant power up to 80%.