Methyl chloride was measured directly in seawater using a purge and trap system and gas chromatography with electron capture detection. The results indicate that surface waters of the northwest Atlantic are a source of methyl chloride to the atmosphere during late spring/early summer. The average surface concentration was 271 pM (&sgr;=68 pM), supersaturated with respect to an assumed tropospheric boundary layer mixing ratio of 0.7 ppbv. Elevated concentrations were observed throughout the region in waters above the seasonal thermocline. Near-surface maxima of differing thickness and intensity were also seen within this upper layer. Broad maxima within the 200--800 m depth range were associated with water masses more recently subducted from the surface than the surrounding main thermocline waters. Coastal inputs of methyl chloride appear not to be an important source in the area of study. Although there is some indication of elevated concentrations associated with higher phytoplankton activity in shelf edge regions, the picture concerning phytoplankton production of methyl chloride is still unclear. Further work is required to look at direct and indirect mechanisms by which phytoplankton may influence the distribution of methyl chloride in the oceans. The global ocean-to-atmosphere flux estimated from this data set, 3.3--4.8¿1010 mol CH3Cl yr-1(1.7--2.4¿1012 g CH3Cl yr-1) suggests that the oceans may contribute a smaller flux of methyl chloride to the atmosphere than previously thought. ¿ American Geophysical Union 1994