An action spectrum was determined for the photoinduced formation of perithecia in a homothallic strain of Nectria haematococca Berk. and Br. var. cucurbitae (Snyder and Hansen) Dingley. Dose-response curves for perithecial formation were obtained from 340 to 510 nanometers at 10-nanometer intervals. Radiation longer than 510 nanometers was not effective for inducing perithecial formation. The action spectrum indicated peaks of activity near 360, 440, and 480 nanometers with shoulders near 420 and 460 nanometers. Minima occurred near 350 nanometers, 390 to 410 nanometers, and 470 nanometers. The general shape of this action spectrum appears to be similar to those obtained for many different near ultraviolet-blue-sensitive organisms in which a flavoprotein molecule was postulated to be the photoreceptor.Previous research has shown that a photorequirement exists for proper development of asexual and sexual reproductive structures in several closely related strains of the fungus Nectria haematococca Berk. and Br. var. cucurbitae (Snyder and Hansen) Dingley (2,8,9,25,33 Baker (2) found that light intensity in particular had a marked effect on the production of the sexual (perithecia) stage. Of three intensities used, the optimal intensity for stimulating the production of perithecia in heterothallic strains was 190 to 195 ft-c.In a more comprehensive study of the light requirement, Curtis (8) found a broad optimal range of intensity for the production of perithecia from 50 to 250 ft-c. Darkness or higher intensities of irradiation greatly inhibited the formation of perithecial primordia and perithecia. In addition, a region of spectral sensitivity in the near UV-blue portion of the spectrum was found for two heterothallic strains and a homothallic strain.