The reproductive response of Rhincalanus gigas to the build up of a phytoplankton bloom in the Southern Ocean was studied during the European iron fertilization experiment (EIFEX). Egg production experiments were conducted over a period of approximately 5 weeks during development of a diatom dominated bloom. R. gigas showed a clear response to increasing chlorophyll a concentrations and the total egg production of the R. gigas population was highest just after the peak of the bloom at day 29 after fertilization. The average peak production was 50 eggs female-1 day-1. The percentage of egg producing females increased from about 0 to 90% during the course of the experiment. Accordingly, the maturation of the gonads reflected the positive response towards enhanced chlorophyll a concentrations. The fast reproductive response indicate that R. gigas was food limited during the period of this study in the Antarctic Polar Front region (APF). © 2006 Springer-Verlag.