The tridacnid clams maintain symbiotic associations with certain dinoflagellates (termed zooxanthellae). Tridacnids are thus candidates to have high tissue concentrations of dimethylsulfoniopropionate (DMSP), a tertiary sulfonium compound that is not synthesized by animals but is commonly produced by dinoflagellates. This study establishes that DMSP is about an order of magnitude more concentrated in the light-exposed and shaded mantle and gills of Tridacna maxima and T. squamosa than in any other known animal tissues. The DMSP concentration in the light-exposed, siphonal mantle--the location of most zooxanthellae--is an inverse function of body size, paralleling an inverse relation between apparent density of zooxanthellae (measured as pheophytin concentration) and body size. The shaded mantle and gills are high in DMSP despite having low densities of zooxanthellae, indicating that high DMSP concentrations occur in molluscan tissue, not just in algal cells. DMSP is almost an order of magnitude less concentrated in the adductor muscle than in other tissues. The high DMSP concentrations found in tridacnids, by providing abundant substrate for formation of volatile dimethylsulfide, probably explain the peculiar tendency of tridacnids to rapidly develop offensive odors and tastes after death: a serious problem for their exploitation as food. Tridacnids are the one group of animals in which DMSP concentrations are high enough in some tissues to be in the range capable of perturbing enzyme function at high physiological temperatures. Thus, tridacnids may require enzyme forms adapted to DMSP.