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Exposure of dark-grown mycelia of Trichoderma viride to white light elicited a transient burst of respiratory activity manifested as increased O2 consumption which was not paralleled by a corresponding increase in CO2 production. The period of increased uptake of O2 lasted for 5–10 min and was independent of the duration of illumination. The inhibitors of respiration tested, antimycin A and mucidin, and the antioxidant, dithiothreitol, effectively suppressed the photostimulated uptake of O2, whereas rotenone, amytal and salicylhydroxamic acid were without effect. It is concluded that the illumination of mycelia caused irreversible photo-oxidation of an as yet unidentified compound, and that the electrons released by the photochemical event were accepted by a NAD-independent flavin dehydrogenase and further transferred to atmospheric O2 via the cytochrome electron-transport chain coupled with the formation of ATP.