Using enzymic and isotope techniques the intracellular partitioning of newly fixed carbon was studied in synchronized cells of Chlamydomonas reinhardtii. Starch and growth metabolism, i.e. the use of carbon in biosynthesis, were found to be the major sinks for photosynthetically fixed carbon in the alga. Sucrose does not accumulate in significant quantities. The amount of carbon partitioned either into starch or growth varies during the 12 hour light/12 hour dark cell cycle. Starch is accumulated at the beginning and at the end of the light period while a net breakdown is observed in the middle of the light period and in the dark. In contrast, nonsynchronized cells accumulate starch all the time in the light which suggests that carbon partitioning is controlled by the cell cycle. Labeled bicarbonate is incorporated into starch even at times when the total intracellular level of starch is decreasing. This indicates a turnover of the starch pool in the light with synthesis and degradation occurring simultaneously and at different rates.