Cells of the axenic strain of the cellular slime mould Dictyostelium discoideum, AX-3, multiply in the liquid nutrient medium HL-5 with a doubling time of 12 h. When the cell concentration reaches approximately 1 X10(7) per ml the rate of cell multiplication begins decreasing and after 20–30 h reaches zero, at a stationary phase cell concentration of 2 to 2–5 X 10(7) cells per ml. The intercept of the extrapolated log phase and stationary phase plots has arbitrarily been considered the onset of the stationary phase. We have found that after cells have been in stationary phase for 24–32 h, mean cell volume increases by 25%, average dry weight by 37%, and average protein content by 24%. These values are close to the expected values for a cell population which is blocked at a point late in the cell cycle. Stationary phase cells also contain 25% more nuclear DNA than log phase cells, indicating that the population of cells at stationary phase is blocked after the DNA replication phase. Finally, when stationary phase cells are washed free of stationary phase medium and reinoculated into fresh medium, they reinitiate cell division synchronously. In the light of the demonstrated relationship between stationary phase and the cell cycle, a possible role for the growth inhibitor produced at stationary phase is considered.