Episodic phytoplankton blooms are part of the natural cycle of production and regeneration in ocean ecosystems. There is growing interest in using iron fertilization to produce massive phytoplankton blooms in parts of the ocean where they do not now occur. The goal is to transfer carbon from the atmosphere to the deep sea (see also the Perspective by Seibel and Walsh in this issue of Science) and trade it in the emerging carbon credit market. In their Policy Forum, Chisholm, Falkowski, and Cullen warn that iron fertilization would be extremely difficult to validate and would significantly alter oceanic food webs and biogeochemical cycles. © 2001 American Association for the Advancement of Science