A theoretical model is presented for the evaluation of the optimal policy of the metabolism of storage materials in unicellular algae, with some experimental testing of the model. Optimality is defined as the maximization of the long term growth rate and/or survival of the population. The main predictions of the analysis of the optimal policy are: (I) The cells divide only during the night. ( ) The energy for division is provided by storage materials produced during the day. Under most conditions, storage materials are produced in the e:cact amount needed to supply the energy required during the night. (3) Storage materials are produced only toward the end of the light period, while the production machinery is synthesized at the beginning of the light period. (4) The weight fraction of storage materials, S, increases when the light intensity increases. The duration of the synthesis of S decreases when light intensity increases, i.e. it starts later in the day. (5) Day length has only a small effect on S and even a smaller effect on the duration of the synthesis. Predictions (1), (3), (4) and (5) have been tested and confirmed experimentally with Chlamydomonas reinhardi. The theoretical and experimental results, and results from the literature, support the optimality hypothesis, and reject the current hypothesis that storage materials are synthesized only when there is a current excess of energy supply. T.O. 1 I 'r.a.