Ultrastructure of conidiophores has been studied in Alternaria solani, pathogenic fungus of potato early blight, in order to elucidate the diffcrence of cytoplasmic structures between the fertile conidiophore and the defifferentiating conidiophore induced by blue light irradiation. In the fertile conidiophore the electron transparent structure developed at the apex and it seems to restrict the cytoplasm to contact with the cell wall of apical dome only at the point of the pore. Concomitantly with the development of the electron transparent structure, a large vacuole developed in the apical cell of the fertile conidiophore. The vacuole is presumed to provide the turgor pressure for protrusion of cytoplasm through the pore to form a conidium initial. Blue light exposure inhibited both the electron transparent structure at the apex and the development of vacuole in the cytoplasm, thus it reverted the fertile conidiophore to the vegetative hypha.