The ultrastructure of the conidiophore and vesicle of Aspergillus flavus Mich, ex Fr. was examined during the processes of maturation and ageing. The conidiophore wall is two-layered, and the outer layer is ornamented with spines. The spines, which seem to be an elaboration of the outer layer, vary in size and shape. The cytoplasm of a young non-septate conidiophore and its apical vesicle is continuous, and filled with ribosomes and other organelles. The organelles are elongated and run parallel to the long axis of the conidiophore. The presence of microtubules and the elongated shape of the organelles suggests an orientated cytoplasmic streaming. On ageing, the wall of the conidiophore and the vesicle increases in thickness, and the cytoplasm appears senescent. While the cytoplasm of the vesicle is hyaline and has a reduced number of organelles, the sterigmata and phialides are actively producing conidia.