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In the sporulation of Alternaria tomato, conidiophores are induced by near ultraviolet irradiation but not by darkness, and the conidia develop only when the irradiation is followed by a period of darkness. Conidial development is suppressed by a short exposure to blue light at a definite time during the dark period following the inductive irradiation. The suppression of conidial development by blue light can be reversed by exposure to near ultraviolet light immediately following the blue light irradiation. This reversion is reversibly suppressed by a further exposure to blue light immediately following near ultraviolet irradiation. Thus, at least two stages are involved in the sporulation of A. tomato, the first being a photochemical stage necessary for the induction of conidiophores, and the second essential for the conidial development which proceeds only in the absence of exposure to the blue region of the spectrum. Moreover, conidial development can be controlled by alternating doses of blue and near ultraviolet light, and the subsequent response depends upon the last kind of radiation given. It is concluded that a new pigment system, which we have named “Mycochrome”, is involved in the blue and near ultraviolet reversible photoreaction, and that this compound plays an important role in the photocontrol of conidial development in this fungus.