Various properties of the starch metabolizing enzymes of Porphyridium purpureum were examined. Phosphorylase was found to possess a Vmax of 12 · 5 × 10−10 μmol G-1-P cell−1 min−1 and a Km of 10 m M, whereas ADPG transferase had a Vmax of 1 · 4 × 10−10 μmol ADPG cell−1 min−1 and a Km of 2 mM. The growth curve of this species was unique under the growth conditions employed in that two exponential phases were observed, each with a considerably different rate of growth (doubling times, td=0 · 4 and 5 · 5 days, respectively). During the first more rapid log phase, there was a net utilization of starch and accumulation of this reserve product was not initiated until the declining growth stage at the end of that phase. Starch accumulation proceeded at a constant rate of 0 · 4 pg cell−1 day−1 through the second exponential phase. Thus the actual rate of starch formation was greatest during the declining growth phase at the termination of the first log phase. The activities of phosphorylase and ADPG transferase increased during the first exponential phase and reached a peak approximately at the time of mostrapid starch formation, whereas amylase activity declined in the same period. Cellular protein contents decreased subsequent to the first exponential phase while total carbohydrates and lipids increased. Chlorophyll a contents also rose and hence photosynthetic rates increased, potentially making more substrates available for starch formation.