Development of oidia, a type of thallic spores from monokaryotic mycelium of Coprinus cinereus was examined with electron microscope and light microscopes. Oidia formation in this fungus is unique in its mode of formation compared with other types of asexual sporogenesis. Oidiogenesis in C. cinereus is carried out in three steps: 1) Formation of oidiophore from the parent mycelium, 2) Formation of initials of oidial cells from swollen oidiophore, 3) Segmentation and detachment of mature oidial cell. Oidiophores appear to spring out singly as a swollen hyphal branches from the normal foot hyphae or sometimes coiled hypha. From the oidiophore, oidial branches sprout out forming a group of $2{sim}6$, most often 4 oidial cells and each oidial cell undergoes a single mitosis resulting in 2 oidia. One of the sibling oidial cells in a group is frequently transformed into a new oidiophore, thus oidiogenic structures are tandemly produced at the several different levels.