An apparatus for automatically exposing small biological specimens to long exposures of monochromatic electro-magnetic radiation, with exposures ranging from a minimum of 10 sec to a maximum of several days or more, essentially consists of: (1) A motor driven disc that holds eighteen specimens and which automatically rotates and stops so that each specimen is irradiated. (2) A timing mechanism which controls the rotation of the disc. This consists of a moving tape (16 mm movie film) in which are punched holes that activate microswitches. The speed of movement of the tape is controlled by different combinations of gears. (3) An automatically operated camera shutter placed directly in front of the monochromator's exit lens, which is coordinated with the rotation of the specimen disc. In addition, a photoswitch is included in the circuit so that the rotation of the specimen disc automatically stops should the monochromator lamp fail. Although this apparatus was designed specifically for monochromatic irradiation of fungi, it would appear to have widespread application for irradiating many forms of biological materials with either electro-magnetic radiation or other forms of radiation.