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The background atmospheric SO: concentration above the sea surface has been established at 0.1 x 10 -6 g/m 3 (35 pptv). Several hypotheses to explain the origin of SO: above the ocean, far from continental sources, have been discussed. We have shown that one of the mechanisms of SO: formation is the biogenic production of reduced organic sulfur compounds in the ocean. Dimethyl sulfide (DMS) has been observed in oceanic areas, with concentrations ranging from 11 to 1150 x 10-9g 1-• in surface waters and from 0.2 to 400 x 10-9g m -3 (0.07 to 144 pptv) in marine atmosphere. This compound is emitted into the atmosphere and oxidized there by photochemically induced processes. That leads partly to SO: formation, and subsequently, to sulfate. This hypothesis is corroborated by a correlation between atmospheric SO: concentrations and the relation between atmospheric SO: concentrations and biological activity. The flux of sulfur by this process (biological activity-, organic sulfides--, SO:-, SO4 =) is estimated to range between 27 and 72 x 10 6 tons of sulfur per year. This biogenic production is comparable to atmospheric sulfur production by sea spray, which is about 44 x 10 6 T $/yr. Therefore, the oceanic production by these two processes could be estimated as being from 71 to 116 x 10 6 T S/yr.