Methyl iodide or iodomethane (CH3I) has recently been registered as a fumigant in many countries, although its environmental impacts are not well understood. Here we report the results of a study on the methylation of mercury by CH3I in natural water by incubation experiments using both Hg ((199)HgCl2 and CH3(201)Hg(+))- and hydrogen (CD3I)-stable isotope addition techniques. We find that methylation of Hg(0), Hg2(2+) and Hg(2+) by CH3I can occur in natural water under sunlight, while only Hg(0) and Hg2(2+) can be methylated in deionized water. We propose that the methylation of Hg by CH3I in natural waters is mediated by sunlight and involves two steps, the reduction of Hg(2+) to Hg(0)/Hg2(2+) and the subsequent methylation of Hg(0)/Hg2(2+) by CH3I. Further quantitative assessment suggests that CH3I-involved methylation of inorganic Hg could be an important source of CH3Hg(+) in an environment where CH3I has been used in large amounts as a fumigant.