During an experiment in the North Atlantic in June 1998, water samples were collected approximately 400 km south of Iceland inside and outside of a bloom of the coccolithophorid Emiliania huxleyi. In vitro dimethylsulphoniopropionate (DMSP) lyase activity (DLA) was quantified using gas chromatography and found to vary from 0.1 to 142.3 nM dimethyl sulphide (DMS) h super(-1). Inside the bloom area the majority of DLA (>74%) occurred in particles >10 mu m, indicating that E. huxleyi (5 to 7 mu m diameter) made only a minor contribution to total DLA. In surface waters, phototrophic dinoflagellates (>10 mu m) made up a high proportion of the total phytoplankton biomass ( approximately 27%) towards the end of the coccolithophorid bloom and may have been the source of most of the DLA. This was also indicated by a significant correlation (p < 0.02) between DLA and the concentration of peridinin, a pigment used as a chemotaxonomic marker for dinophytes. The data presented here are the first field measurements of DLA in a coccolithophorid bloom and suggest that even a relatively low concentration of photosynthetic dinoflagellates larger than 10 mu m (during our study 18 to 105 cells ml super(-1)) may contribute significantly to DMS production. Although dinoflagellates are recognised as an important source of particulate DMSP, recognition of their significance for DMS production in the field has previously been limited to a few observations in highly concentrated coastal and shelf blooms. Very little information exists on DLA in dinophytes and further investigations are warranted in order to improve our understanding of the biogeochemical and ecophysiological significance of DMSP lyases in this group of phytoplankton.