The dramatic cytological and biochemical changes in Chlorella pyrenoidosa, occurring immediately prior to and during the period of nuclear division and cross-wall deposition, seem to demand an increase in (a) the supply of carbon building blocks for organic synthesis, and (b) the energy-generating capacity of the cell during this stage of development. The fact that the photosynthetic rate approached its lowest level of the cell cycle during this stage of cellular development, and that at the same time the endogenous respiration rate accelerated, leads to the inference that a carbohydrate, lipid, or similar carbon and/or energy reserve accumulated prior to and was mobilized and utilized during the period of nuclear division. In order to determine whether starch could serve as such a carbohydrate reserve; the total cellular starch and amylose, of a high temperature strain of Chlorella pyrenoidosa, was measured during two consecutive synchronous growth cycles in continuous light. Although the rate of starch accumulation exhibited dramatic periodism, when expressed on a cellular phosphorus or dry weight basis, little periodism in the rate of accumulation of this cellular fraction was observed per ml of culture. On a per ml of culture basis, the cellular starch increased continuously, except for a one-hour period of slight net utilization during cell division (cell release) in both cycles and a one hour period of similar net utilization at the beginning of the second cycle. Amylose remained at approximately 30 percent of the total starch throughout both cycles. It was concluded that starch undoubtedly plays a more significant role in the maintenance of the cellular economy than was indicated by its only slight periodism in the cellular level during the cell cycle. Thus, the need for future C¹⁴-labeling studies to measure the turnover rate, of the starch and related cellular fractions during the cell cycle was made vividly / M.S.