Dynamics of metazooplankton populations and the impact of grazing on their potential food supply was studied in an in situ iron fertilization experiment (EisenEx) in the Antarctic Circumpolar Current. Copepod nauplii, copepodites and the adults of small copepods (< 1.5 mm) were sampled with Niskin bottles (12 L). The exceptionally high concentrations we recorded (average 42 nauplii L-1 and 24 copepodites and adults L-1) suggest that vertical net hauls seriously undersample these organisms. During the experiment copepod nauplii numbers stayed more or less constant. Abundances of copepodites and adults of small copepods increased threefold indicating either enhanced recruitment from naupliar and copepodite stages respectively, and/or congregation within the patch due to migration of individuals from below the mixed layer. In either case the copepod population appeared to be food-limited outside the patch. Copepod grazing apparently suppressed protozooplankton stocks: Heterotrophic dinoflagellates and aloricate ciliates increased with food supply but decreased subsequently. Tintinnids decreased but empty loricae increased twofold indicating heavy grazing mortality. Thus interactions between grazer populations facilitated population growth of diatoms which dominated the iron-induced diatom bloom.