Chlorophyll-protein complexes and electron transport activities were measured during iron nutrition-mediated chloroplast development in sugar beet (Beta vulgaris L. cv F58-554H1). Results showed that the chlorophyll-protein complexes associated with the reaction centers of photosystem I (CP1) and photosystem II (CPa) and the electron transport activities of these two photosystems per leaf area increased rapidly during the first 24 to 48 hours of iron resupply to iron-deficient sugar beet plants. Bulk chlorophyll and the amounts of light-harvesting chlorophyll-proteins increased after a lag period of 24 hours. The changes in chlorophyll-proteins with time were apparently the cause of an initial increase, then decrease, in the chlorophyll a/b ratio during iron resupply. There was evidence that iron deficiency diminished photosystem I more than photosystem II. We propose that there are two distinct phases in iron nutrition-mediated chloroplast development: (a) the commencement of the synthesis of the lipid matrix of the thylakoid membrane, including a fully functioning electron transport (and photosynthetic) system, during the first 24 hours of iron resupply; and (b) after 24 to 48 hours, the formation of the bulk of the thylakoid proteins, including the light-harvesting chlorophyll-proteins with which the large increase in total chlorophyll is associated.