The role of biogenic dimethylsulfide from the ocean surface of atmospheric sulfur has so far been largely based on the results of mass balance calculations and on a small number of determinations of DMS in seawater. On a cruise of the R/V Meteor across the Altantic from Hamburg, F.R.G., to Montevideo. Uruguay, we have sampled the surface ocean at short intervals and analyzed over 300 seawater samples aboard ship immediately after collection. Simultaneously, the atmospheric concentration of DMS has been measured at 13 m above the ocean surface. Biological and chemical oceanographic data have been collected during the cruise to aid in the interpretation of the findings on DMS. A number of vertical profiles to the bottom of the ocean were analyzed for DMS and other parameters. The concentrations of DMS in the surface ocean ranged from 17.7 to 743 ng S(DMS)/1, with a mean of 91 ng S(DMS)/1(n=231) for the area traversed. This mean is significantly higher than previous work had suggested. The highest concentrations of DMS were found in productive regions on the continental shelves, in marginal seas and in some estuaries. The vertical distribution of DMS is closely related to the distribution of primary productivity, as measured by chlorophyll and nutrient concentrations. Low concentrations (≤10 ng S(DMS)/1) were found in the deep ocean. From these distributions and from our work on pure cultures of marine algae we conclude that marine primary producers are the dominant source of DMS to seawater and consequently to the atmosphere. The concentrations of DMS in the atmosphere ranged from 2 to 44 ng S(DMS)/m3. The mean concentration of 6.1 ng S(DMS)m3 is significantly lower than postulated concentrations of 40--200 ng S(DMS)/m3 for marine background atmosphere, thus questioning the rates and mechanisms given for reactions of DMS in this environment. Model calculations for the transfer of DMS across the air-sea interface, using the data on DMS in seawater and the concentrations of DMS in the atmosphere, suggest a global flux of 34--56¿1012g of sulfur in the form of DMS per year from the oceans to the atmosphere.