Cell growth and division of the ciliateTetrahymena corlissi were examined upon refeeding after prolonged starvation of up to 12 days. Division did not automatically occur when a certain critical cell size was reached. Rather, it varied both with the nutritional history of the cell and the nutrient conditions in which the cell was growing. Upon refeeding, cells starved for 12 days divided at a smaller size and later than cells starved for 6 days. Cells refed at high density took longer to begin division than cells refed at low density. The results are discussed with respect to the "relative starvation" and "critical constituent" models of the cell cycle and in terms of the polymorphic life cycle ofTetrahymena species.