Depth profiles of dimethylmercury (DMHg) concentration were determined at nearshore to offshore sites in Monterey Bay, California. The onset of spring upwelling in the bay was accompanied by increases in DMHg concentrations. Profiles show DMHg increasing gradually with depth in fall and winter from <0.03 pM at the surface to 0.5 pM at 200 m. During the spring, DMHg concentrations increased between 30 and 100 m, first within Monterey Bay, then offshore. This change was accompanied by an increase in DMHg concentrations in the surface water DMHg between fall/winter (<0.03 pM) and spring (0.06-0.29 pM). Microbial activity associated with the remineralization of sinking organic matter produced by the high primary production in the bay may result in the relatively high DMHg in subsurface water in the bay, which when upwelled may facilitate the incorporation of organomercury into biota. As a result, productive coastal upwelling areas may represent an important source of methylated mercury to surface waters, and thus be an important source of mercury to marine ecosystems.