Dimethyl sulphoxide (DMSO) represents a major pool of dissolved dimethylated sulphur in seawater. However, the origin and fate of this compound in the marine environment, and its role in the biogeochemical cycle of dimethyl sulphide (DMS), remain unclear. The only established route for the formation of DMSO in oxygenated seawater is photochemical oxidation of DMS. It is not known whether significant biotic production pathways exist. In a study of methylated sulphur speciation in coastal North Sea waters and cultures of marine unicellular algae, we measured pools of particulate DMSO (DMSO(p)) at nanomolar and micromolar concentrations, respectively. Analyses of size-fractionated seawater particulates and incubation experiments provided evidence that DMSO(p) was associated with microplanktonic organisms. Log-phase cultures of Amphidinium carterae and Emiliania huxleyi exhibited intracellular dimethylsulphoniopropionate (DMSP) to DMSO molar ratios of 25 and 8, respectively. Our results strongly suggest the existence of biological production and release of DMSO in eukaryotic microplankton.