1. A grating spectroirradiater (biological spectrograph) was designed and assembled for use in monochromatic light irradiation experiments on relatively large biological specimens, i. e. higher plants, in a controlled environment.<BR>2. A short-arc xenon lamp (6.5 kW) was used as the light source. The dispersing element was a plane diffraction grating with a ruled area of 154×206 mm (1, 200 lines/mm, blazed at 600 nm) . The usable wavelength region was extended from 350 nm to 800 nm. A monochromator of the MONK-GILLIESON Mount type was used to obtain monochromatic light flux at as high an energy level as possible. Maximum intensity of the flux was 20, 000 erg/cm²/sec.<BR>When entrance and exit slit widths were set at 0.5 mm and 5 mm, respectively, the half bandwidth of monochromatic light was 0.95 nm at 398 nm and 2.4 nm at 791 nm.<BR>3. To simultaneously irradiate each specimen with monochromatic light beams of the same intensity and the same spectral bandwidth, relationships between the entrance slit width and exit slit widths, and the distance between the exit slit and specimen were examined. Conditions for setting them at appropriate values were studied.<BR>4. To make the intensity distribution of monochromatic light on the irradiation plane uniform, a dispersing mirror composed of small convex mirrors arranged on a concave surface was contrived.<BR>5. Stray light in the monochromatic light as measured with a double beam spectroradiometer was less than 10^-5.<BR>6. Air temperature and relative humidity in the laboratory were controlled in ranges of 15°C-30°C±1°C and at 65%±5%, respectively.