SUMMARYThe development of conidiophores and botryose solitary blastospores was studied in Oedocephalum roseum by light and electron microscopy, and for convenience was divided into several stages. Development was a continuous process with no pause at any stage, although increase in volume of the sporing structure was much reduced during production of the denticles. Cytochemical light microscopy supported electron microscopic observations of distribution of organelles and storage substances in the sporing structure at each stage of development. A Spitzenkorper, comprising two types of unit memhrane-bounded vesicles of different size, was present at the tip of the growing conidiophore. At the time of blow-out of the denticles, small vesicles, thought to contain wall-softening enzymes, accumulated in groups around the periphery of the ampulla, and the wall in these regions stretched, becoming thinner, and bulged to form the wall of the denticle initials. Exposed sulphydryl groups were detected in the wall of these bulges. The denticles grew to become tubular, and spherical conidium initials formed at their tips. Electron microscopy showed that membrane-bounded vesicles were present in all regions of growth and development, and to the exclusion of all other organelles in the tip of the conidiophore, in denticle blow-outs, in conidium initials and in tips of elongating conidia. In these regions much RNA was detected. The sequence of first occurrences of organelles and storage substances from the young conidiophore tip backwards was also the sequence of their entry into the developing conidia, nuclei being the last of the organelles to enter. During development and growth the wall around the whole sporing structure was not broken at any stage. However, there was a succession of wall layers in each structure during its development, new layers being formed internally and being progressively displaced towards the outside. The new inner wall layer formed by the hypha at the inception of the conidiophore gave rise to the three outer layers of the conidial wall, the inner layer of the conidium being a continuation of the inner layer of the ampulla wall, which was formed before denticle production. Blastospore formation resembled the process of budding in yeasts, in that vesicles probably containing wallsoftening enzymes were present in the region of denticle blow-out and of budding, and the walls of the denticle and of the yeast bud were made up of parent cell wall material.