THE eastern equatorial Pacific Ocean is one of only three open-ocean regions where low phytoplankton chlorophyll biomass persists despite perennially high nitrate and phosphate nutrient concentrations’. In 1993, an area within this region was artificially enriched with a single dose of soluble iron to test whether phytoplankton are physiologically prevented from utilizing the available nutrients by the low natural iron concentrations(2,3). Although photosynthesis was stimulated(4), the observed lack of a bloom or a significant decrease in nutrient concentrations could not he attributed unequivocally to zooplankton grazing(5-7), further iron limitation or secondary nutrient limitation(2,4). In 1995, a second iron-enrichment experiment (IronEx II) was conducted in which the same total dosage of iron was added, but over eight days(8). A massive phytoplankton bloom developed, significantly reducing surface-water nutrient and CO2 concentrations(8-10). Here we report in situ measurements of fluorescence during IronEx II, which shelf that the iron enrichment triggered biophysical alterations of the the phytoplankton’s photosynthetic apparatus, resulting in increased photosynthetic capacities throughout the experiment and, hence, the observed bloom. These results unequivocally establish physiological limitation of phytoplankton by iron as the cause of the high-nitrate, low-chlorophyll phenomenon in this ocean region.