Phytoplankton are the primary producers of the sulfur-containing compatible solute dimethylsulfoniopropionate (DMSP). These cells are consumed by mesozooplankton, which, in turn, may be eaten by marine vertebrates. From the gut of one such animal, the Atlantic Herring Clupea harengus, we isolated strains of the ?-proteobacteria Pseudomonas and Psychrobacter that grew on DMSP as sole carbon source, and which produced the environmentally important sulfurous volatile dimethyl sulfide (DMS). In both bacterial genera, this ability was because of the previously identified gene dddD, which specifies an enzyme that liberates DMS from DMSP. DMS production was stimulated by pre-growth of cells on the substrate DMSP. This is the first identification of DMSP-degrading bacteria and their relevant genes in the gut microflora of any vertebrate.