We describe the design and operating characteristics of an optically controlled varactor diode. Applications include optically controlled microwave elements and detection of optical signals in lightwave receivers. The structure consists of a p-n junction formed in an epitaxial layer of InP grown by metalorganic chemical vapor deposition on a semi-insulating InP substrate. A capacitance shift as high as 1.5 pF has been achieved for a photocurrent level as low as 9 μA.