Methyl iodide concentrations have been measured in air samples, surface, and subsurface waters of the NW Atlantic, NE Atlantic, and Pacific Oceans. They are shown to be substantially oversaturated in all surface waters. Our best estimate of methyl iodide flux from the ocean to the atmosphere is 0.9--2.5¿109 mol yr-1, but further substantial and unquantifiable uncertainties exist because of the limited spatial and temporal data on which this flux estimate is based. Pronounced subsurface maxima in methyl iodide concentrations seen in the Pacific Ocean and the Sargasso Sea are thought to result from production and accumulation of the gas in the relatively poorly ventilated waters beneath the surface mixed layer. Our calculations suggest that latitudinal variations in methyl iodide concentrations in the Pacific Ocean are partly controlled by temperature-dependent chemical loss. ¿ 1999 American Geophysical Union