IT has long been hypothesized that iron concentrations limit phytoplankton productivity in some parts of the ocean(1-3). As a result, iron may have played a role in modulating atmospheric CO2 levels between glacial and interglacial times(4), and it has been proposed(5) that large-stale deposition of iron in the ocean might be an effective way to combat the rise of anthropogenic CO2 in the atmosphere. As part of an experiment in the equatorial Pacific Ocean(6), we observed the effect on dissolved CO2 of enriching a small (8 x 8 km) patch of water with iron. We saw significant depression of surface fugacities of CO2 within 48 hours of the iron release, which did not change systematically after that time. But the effect was only a small fraction (similar to 10%) of the CO2 drawdown that would have occurred had the enrichment resulted in the complete utilization of ail the available nitrate and phosphate. Thus artificial fertilization of this ocean region did not cause a very large change in the surface CO2 concentration, in contrast to the effect observed in incubation experiments(3), where addition of similar concentrations of iron usually results in complete depletion of nutrients. Although our experiment does not necessarily mimic all circumstances under which iron deposition might occur naturally, our results do not support the idea that iron fertilization would significantly affect atmospheric CO2 concentrations.