Labeling with the thymidine analog 5'-bromodeoxyuridine has been used to follow cell cycling during development of the cellular slime mould Dictyostelium discoideum. The results show that cycling continues during development and that 75% of all cells from axenically grown cultures go through S-phase. Although cell cycle progression is temporarily blocked in all cells during early aggregation, it is specifically reinitiated in prespore cells later in development. Prestalk cells do not reenter the cycle. Pulse chase experiments indicate a close correlation between cell cycle phase at the onset of development and differentiation fate. Cells in S-phase and early G2 differentiate preferentially into prestalk cells; cells in the later part of G2 differentiate into prespore cells.