Ultrastructure of conidiophore has been studied in Alternaria solani, the pathogenic fungus of potato early blight, through the observation of the transverse sections as well as the longitudinal sections of the first cell of the fertile conidiophores and the blue light-irradiated, dedifferentiated conidiophores. The doughnut-shaped ETS at the apical dome and myelin figures closely associated with it were observed. The large vacuole developed in the middle part of the cell. Between the ETS and the vacuole many mitochondria mixed with the endoplasmic reticula converged. Nuclei were located below the large vacuole. Thus, the zonation of protoplasmic components was recognized in the first cell of the fertile conidiophore. In the later stage of the developmental stage a blue light irradiation induced to disperse the large vacuole and the conidiophore neither formed a conidium nor reverted to a vegetative hypha. The blue lightirradiated conidiophore at an early stage showed disorganization of the zonation of protoplasmic components, resulting in the vegetative regrowth of the conidiophore.