In general, growth and differentiation are mutually exclusive, but they are cooperatively regulated during the course of development. Thus, the process of a cell's transition from growth to differentiation is of general importance not only for the development of organisms but also for the initiation of malignant transformation, in which this process is reversed. The cellular slime mold Dictyostelium, a wonderful model organism, grows and multiplies as long as nutrients are supplied, and its differentiation is triggered by starvation. A strict checkpoint (growth/differentiation transition or GDT point), from which cells start differentiating in response to starvation, has been specified in the cell cycle of D. discoideum Ax-2 cells. Accordingly, integration of GDT point-specific events with starvation-induced events is needed to understand the mechanism regulating GDTs. A variety of intercellular and intracellular signals are involved positively or negatively in the initiation of differentiation, making a series of cross-talks. As was expected from the presence of GDT points, the cell's positioning in cell masses and subsequent cell-type choices occur depending on the cell's phase in the cell cycle at the onset of starvation. Since novel and somewhat unexpected multiple functions of mitochondria in cell movement, differentiation, and pattern formation have been well realized in Dictyostelium cells, they are reviewed in this article.