Intensive measurements of atmospheric methyl iodide taken at high, middle, and low latitudes over a period of 3 years have provided evidence for its photochemical production in seawater and given new information that sea-to-air transport of CH3I is mainly controlled by surface seawater temperature (SST). These findings suggest a highly localized production and distribution of CH3I in the surface microlayer. As a result, the oceanic emission of CH3I is likely to be larger than previous estimates based on the classical two-layer model. Owing to the SST dependence of atmospheric CH3I concentration, its impact on tropospheric or stratospheric ozone depletion would be increased by El Nino or future global warming. ¿ 2001 American Geophysical Union