Concentrations of biogenic dimethyl sulfide (DMS) in seawater around mainland Britain in winter and summer (1985) ranged from 1 to 1,100 ng S (DMS) liter-‘. The mean winter DMS concentration was 4 ng S (DMS) liter-’ compared with the mean summer concentration of 220. Analyses of phytoplankton species composition in summer indicate that the main sources of DMS were coccolithophores, various dinoflagellates including the bloom species Gyrodinium aureolum, and certain unidentified taxa of small flagellates. Concentrations of dimethylsulfoniopropionate (DMSP), the precursor of DMS, were measured in 53 of the summer samples, and its mean concentration was about an order of magnitude greater than that of DMS. Particulate (>0.2 pm) and dissolved fractions of DMSP were operationally resolved, with the latter showing the stronger correlation with DMS. Preliminary estimates for the areal and temporal average flux of sulfur (DMS) from the North Sea to the atmosphere during summer are of the order of lo3 pg S m-2 d-l, a 60-fold increase over winter flux. Biogenic emission in summer is equivalent to about 16% of the spatially averaged anthropogenic emission from Europe.