THE surface waters of the equatorial Pacific have unusually high nitrate and phosphate concentrations, but relatively low phytoplankton biomass(1-3). This high nitrate, low chlorophyll’ (HNLC)(4) Phenomenon has been ascribed to ’top-down’ grazing pressure by herbivores, which prevent the phytoplankton from fully utilizing the available nutrients(5). In the late 1980s, however, Martin and co-workers proposed that iron, which is delivered to the remote open ocean in aeolean dust(6), is he key factor limiting the standing crop of phytoplankton in HNLC areas(7,8). Using a sensitive fluorescence method(9), we have followed changes in photochemical energy conversion efficiency(9,10) of the natural phytoplankton community both before and after artificial enrichment with iron of a small area (7.5 x 7.5 km) of the equatorial Pacific Ocean(11). Our results show that iron limits phytoplankton photosynthesis in all size classes in this region bg impairing intrinsic photochemical energy conversion, thereby supporting the hypothesis of physiological (’bottom up’) limitation by this element.