THE concentrations of bioavailable iron in the surface waters of some ocean regions may indirectly modulate climate by controlling phytoplankton productivity and thus the amounts of carbon dioxide(1) and dimethyl sulphide (DMS) that are exchanged with the atmosphere. Oxidation of DMS is involved in the formation of atmospheric sulphate particles, which can exert a climate cooling effect(2) directly (by scattering and absorbing solar radiation), and indirectly (by affecting cloudiness and hence global albedo), But direct evidence supporting the hypothesis that DMS production in the ocean is affected by iron availability is lacking. Here we report changes in the concentrations of DMS in response to in situ iron-enrichment during two ecosystem-scale experiments designed to investigate the biological and chemical effects of iron fertilization of under-productive surface ocean waters(3,4). The first such experiment revealed a limited overall biological response(3) and no significant changes in DMS concentrations, although the concentrations of its biochemical precursor doubled(5). The second experiment, designed to better mimic the natural process of iron enrichment, elicited a much stronger biological response(4), and DMS concentrations increased by a factor of 3.5. This result provides direct support for an important link in the iron-DMS-climate hypothesis.