A series of 8 methyl iodide (CH3I) production experiments were conducted during Meteor cruise 55 to the tropical Atlantic. Natural seawater samples were incubated in quartz flasks, on-deck, for 24 hours both in the dark and under the natural light-dark cycle. A subset of incubations was conducted with samples that had been either filtered or poisoned to reduce or eliminate within-flask biological activity. The accumulation of CH3I in the dark incubations was <20% of the 'light' production. The experimental results supported prior inferences that CH3I is produced in the open ocean by a light-dependent production pathway that is not directly dependent on biological activity. The average observed net production rate in full sunlight incubations was 2--5 nmol m-3 day-1. A mass balance for the tropical Atlantic surface layer implies that this rate of production can support at least 50% of the average sea-to-air flux of 23 nmol m-2 day-1.