Dimethylsulfoniopropionate (DMSP) synthesized by marine phytoplankton is the principal source of dimethylsulfide (DMS), an important climate-affecting gas. Grazing by small zooplankton on phytoplankton often accelerates DMS production from algal DMSP. The effects of grazing by benthic suspension feeders, such as bivalve molluscs, however, have not been studied, even though their populations sometimes process a sizable fraction of local phytoplankton production. We fed Tetraselmis sp. Strain UW474 (27 to 42 fmol DMSP cell–1) to adult mussels Mytilus edulis and scallops Argopecten irradians and studied the fate of the algal DMSP during the 24 h following ingestion. Almost none of the ingested DMSP reappeared in the environment as DMS or DMSP; the amount that appeared in the ambient water as DMS was <1% of that ingested, and the sum total that appeared either as fecal DMSP (which microbes might convert to DMS) or in the water as DMS or DMSP was ?3 to 4% of that ingested. In the short term, therefore, thriving bivalve populations probably strongly reduce the rate of DMS formation (direct or indirect) from local algal DMSP, in contrast to zooplankton populations. Ingested DMSP is likely accumulated in the bodies of mussels and scallops. However, although we have weak evidence of partial accumulation in scallop gastrointestinal tissue, we were unable to document accumulation in mussels because of high variability and statistical nonnormality in their naturally occurring DMSP content. In total, we showed that in the 24 h following feeding, mussels and scallops do not facilitate ambient DMS formation from algal DMSP and evidently sequester most of the algal DMSP they ingest.