ABSTRACT: During the IronEx II experiment in the eastern equatorial Pacific (May to June 1995), the response of the microplankton community to mesoscale iron fertilization was followed using a combination of marker-pigment, microscopical and flowcytometric techniques. Phytoplankton standing stock bloomed dramatically over a period of 6 d following 3 iron additions of 2 and 1 nM, respectively. Carbon biomass in the patch increased by a factor of 4, chlorophyll a by about a factor of 16 anddiatoms by >70 fold relative to contemporaneous levels in the ambient community. The bloom then plateaued sharply and remained at a more or less constant level for 4 d, despite the addition of more iron (1 nM) and physiological indices (low C:chla ratio and elevated photochemical quantum efficiency) suggesting that the cells were healthy and growing rapidly. Relatively large pennate diatoms (Nitzschia spp., median length 20 to 24 µm) dominated the patch bloom, with smaller pennatespecies and centric diatoms declining in relative importance. Heterotrophic bacteria increased at a slow rate (0.08 d^-1) for >10 d during the experiment, as did heterotrophic nanoflagellates. There were also indications of enhanced cellsize, cellular pigment content and possibly growth rates of small phytoplankton. Nonetheless, little difference was observed between the ambient community and the peak patch bloom with respect to the size composition of auto- and heterotrophic populations10 µm in cell size. The relative constancy of the smaller size fractions, the sharp curtailment of net growth of the bloom after 6 d, and >3-fold increase in large heterotrophic dinoflagellates and ciliates suggest that protistan grazers may haveplayed an active role in controlling the phytoplankton response to increased iron availability.