An enzyme assay for measuring dimethylsulfoniopropionate (DMSP)-lyase activity was applied to natural sea water samples taken during the 1993 spring bloom off the Dutch coast. When relating the measured DMSP-lyase activity to the most abundant algal species found during the cruise, a significant correlation with Phaeocystis sp. numbers (r(2) = 0.9660, n = 23) was observed, but not with any of the other species present, or with total diatom numbers, total diatom biovolume or total protein. The calculated dimethyl sulfide (DMS) production by Phaeocystis sp. in a typical spring bloom off the Dutch coast appeared to be 1.5 to 4.5 times higher than the DMS loss due to flux to the atmosphere and photochemical oxidation. The results indicate that the alga Phaeocystis sp. has a very active DMSP-lyase, specific for this species, that was potentially responsible for the conversion of DMSP to DMS in the area investigated, leaving 35 to 75% of DMS for bacterial consumption.