Production and decomposition of DMSP and DMS in a microbial food web were investigated by means of a 5‐d incubation of prescreened seawater samples (<1, <10, <100 µ m) from northwestern Mediterranean coastal waters. The major goal of size fractionation was to create predator‐free compartments. Aplastidic flagellates, plastidic nanoflagellates (<10 µ m), or both contributed to the particulate DMSP pool, but DMSP was found to be predominantly associated with the populations of small dinoflagellates (~20 µ m, Prorocentrum sp., Gymnodinium sp.). Particulate‐DMSP values and free‐cell counts in the < 100‐ µ m fraction, however, were not significantly correlated. This result reflects the grazing pressure of ciliates on the DMSP carriers since significant amounts of DMSP can be stored temporarily by the microzooplanktonic predators. The initial level of soluble DMSP was 12% of the total DMSP pool of seawater and less than 2% at the end of the experiment. This result suggests that free DMSP is not produced in significant amounts by the predator‐prey interactions in the microbial food web. Conversely, DMS was shown to be excreted following ciliate grazing on dinoflagellates and DMSP decomposition. Mass balance between DMSP and DMS was achieved after a time lag of ~1.5 d. We suggest that DMSP decomposition into DMS in the marine environment can occur through microplanktonic catabolism by two pathways: enzymatic cleavage and formation of an intermediate compound after demethylation. Overall, our work indicates that microzooplankton grazing is significant in DMS release in open‐ocean, surface waters.