A suite of naturally produced volatile halomethanes which are potential sources of gaseous halogens to the atmosphere have been measured in the water column of the NW Atlantic Ocean. Bromoform, chlorodibromomethane, dichlorobromomethane, dibromomethane, and methyl iodide all showed higher concentrations in coastal waters than in the pelagic zone. Such a distribution is consistent with known sources of these compounds in macro-algae. Chloroiodomethane alone showed elevated concentrations in surface open ocean waters, and its distribution is interpreted as indicating production by phytoplankton, either directly or through an intermediate such as di-iodomethane. The latter explanation is supported by post-cruise estimates of the di-iodomethane distribution. Whereas most of the compounds measured in this study have higher concentrations in the upper water column, both dibromochloromethane and bromodichloromethane showed increases with depth which would be consistent with a slow reaction of surface-derived bromoform with chloride. ¿ American Geophysical Union 1993