Heat-shock of macroconidia of Neurospora crassa at 46 degrees C followed by shift-down to 25 degrees C determines premature conidiogenesis. The nuclei and cytoplasmof heat-treated, swollen conidia contain spots of a dense material especially concentrated around the nucleolus in short time treated ones. In the first proconidium apically budding on the enlarged tip of the premature conidiophore, small vesicles are later seen lining the initially simple septum separating the proconidial units into conidia. The doubling of this interconidial septum is surface viewn as a thick annulus. Disarticulation of the conidial untis intervenes along a septal furrow of electroluscent material. Interconidial continuity through the septal proes is transiently insured by a connective which is ruptured for final liberation of the conidia.