The effects of iron deficiency and iron reconstitution on the ultrastructure of the unicellular cyanobacterium Anacystis nidulans R2 were studied by electron microscopy. Low-iron cells, grown with different amounts of aeration, were analyzed at 6, 12, and 24 h after the addition of iron. Low-iron cells had a decrease in the quantities of membranes, phycobilisomes, and carboxysomes and a large increase in glycogen storage granules. In cells aerated with gentle shaking, the addition of iron caused the number of carboxysomes to increase rapidly within 6 h. This was paralleled by a decrease in the quantity of glycogen storage granules. Carboxysomes were associated with the nucleoplasmic face of the inner photosynthetic membrane in normal, but not low-iron, cells; they once more contacted the membrane by 6 h after iron addition. Phycobilisome assembly was apparent by 6 h, and the number of phycobilisomes increased throughout reconstitution. Membrane restoration was accomplished in two stages: (i) components were added to preexisting membranes until about 12 h, and (ii) new membranes were synthesized beginning at 12 to 18 h. Low-iron cells grown by bubbling with air had only one to two concentric layers of membrane per cell. The addition of iron led to a pattern of reconstitution that was similar to that described above with two important exceptions. Under these conditions, the number of carboxysomes remained low and the carboxysomes rarely contacted the photosynthetic membranes. New membranes were not synthesized until the culture had reached the late-logarithmic growth phase and after all other morphological features had returned to normal.