Upon starvation, cells of the simple eukaryote Dictyostelium discoideum aggregate and differentiate into several cell types. Two main cell types are prestalk and prespore, which later usually become stalk and spore cells. The differentiation is plastic, and several factors can alter cell-type ratios. Two mechanisms have been proposed to regulate the initial cell type. We and others have proposed that cell type is initially determined by cell-cycle phase at the time of starvation: prestalk cells are derived from cells which, are the time of starvation, happen to be in a roughly 2-hr-long sector of the cell cycle which overlaps S and early G2 and that certain extracellular factors are then used to maintain the proper prestalk:prespore ratio and to control later stages of development such as the prestalk-to-stalk conversion. To examine the relationship between initial cell-type choice and the cell cycle, and how this 2-hr-long sector is generated, we increased the length of S phase by mild treatments of cells with DNA-synthesis inhibitors. When the fraction of the cell cycle occupied by S phase is increased and the cells are then starved, the prestalk:prespore ratio increases. This increase was observed using two markers for prestalk cells, CP2 and ecmA::lacZ. In addition, there is a close correlation between the fraction of the cell cycle occupied by S phase and the prestalk:prespore ratio, irrespective of total cell-cycle length. These results validate the hypothesis that the initial choice of cell type is determined by cell-cycle phase at the time of starvation, and indicate that the cell-type choice mechanism monitors the cell cycle rather than using an independent 2-hr-long timer started at the beginning of S phase.