The in situ variation of dimethyl sulfide (CH3SCH3, DMS) at a fixed station in a coastal area of the Baltic Sea has been studied for a period of time (January 1987 to June 1988) covering the annual biological cycle. DMS in the surface waters of the brackish Baltic Sea showed a clear seasonal variation, ranging from 2 to 200 ng S L-1. Lowest concentrations were in winter, peak values followed the spring bloom, and a pronounced maximum was found during the summer (July--August). Concentrations above low winter levels occurred only when the trophogenic layer was depleted of inorganic nitrogen. From our data it is clear that the seasonal variation in DMS concentration is related to biological activity. However, we did not find any correlations between DMS concentration and gross parameters such as chlorophyll a, total phytoplankton biomass, or primary production on an annual basis. Further, we were not able to relate high DMS concentrations to any particular phytoplankton species or species assemblages. It appears that DMS production is primarily associated with phytoplankton growth under nitrogen-limited conditions and not with certain species. We found a significant correlation of ambient DMS concentration with copepod and total zooplankton biomass, suggesting zooplankton grazing pressure as the major factor responsible for the liberation of β-dimethylsulfoniopropionate (DMSP) from phytoplankton cells and thus for the DMS production. The turnover time of DMS in the water column was calculated to be of the order of 2 days, and the most effective sink process seems to be of microbiological and/or chemical origin. Previous to this study, no insitu data set has been available to test the relative importance of the various factors responsible for the DMS production in seawaters. We have demonstrated that variations in DMS concentration must be looked upon as the result of complex physiological as well as ecological interactions. ¿ American Geophysical Union 1990