A preparation of pure protoplasts of Geotrichum candidum became osmotically stable and colonies developed when the protoplasts were embedded in stabilizing thin-layer-agar and incubated with stabilizing basal medium. When growing protoplasts were exposed to distilled water and then reincubated with basal medium, the process of regeneration of protoplasts could be quantitatively demonstrated by counting colonies. The process was divided into three phases, lag, logarithmic, and stationary. Furthermore, the state of regeneration of protoplasts at each phase could be seen in detail by microscopic studies of protoplasts under similar growth conditions. In the lag phase, which lasted for 2 hr after inoculation, protoplasts were completely destroyed when placed in distilled water. During the logarithmic phase, from 2 to 5 hr after inoculation, protoplasts rapidly became osmotically stable and about 18% of them were growing. In the stationary phase, most protoplasts developed germ tubes within 2 hr. These results suggested that there are two main phases, although individual cells passed through three different conditions, osmotically labile, osmotically stable, and growing. No apparent structure of cell wall material could be detected by electron microscopy on the surface of the membrane of these osmotically stable cells.