Abstract Vertical and horizontal profiles from the North and South Pacific Oceans demonstrate,the existence of three species of dissolved selenium: selenite, selenate, and organic selenide (opera- tionally defined). In surface waters, organic selenide makes up about 80% of the total dissolved selenium, selenite concentrations are uniformly low, and selenate concentrations rise with increased vertical mixing. The organic selenide maximum,(thought to consist of seleno-amino acids in peptides) coincides with the maxima of primary productivity, pigments, bioluminescence, and dissolved free amino acids. Deep ocean waters are enriched in selenite and selenate, while organic selenide is nondetectable. In suboxic waters of the tropical northeastern Pacific, organic selenide concentrations rise, while selenite values decrease. The downward flux of particulate selenium generally decreases with depth, and fluxing particulate selenium is found to be primarily in the (-2) oxidation state. These data allow a re-evaluation of the internal biogeochemical,cycle of selenium. This cycle includes selective uptake, reductive incorporation, particulate transport, a multistep regeneration, and kinetic stabilization of thermodynamically unstable species. The,marine,biogeochemical,cycles,of many,trace elements,include,the processes of uptake from dissolved to particulate form, particulate transport, and regeneration back to the dissolved,state. Uptake,can occur pas- sively by adsorption,onto,particle surfaces and coprecipitation into solid phases, or ac- tively by,selective incorporation,into bio- logical tissues and,skeletal,material. The vertical transport,of trace elements,from,the surface zone,to the deep,sea via detrital mat- ter is a function,of the type,of carrier and of sinking,rates. The regeneration,of a par- ticulate-bound,trace,element,to,the,dis- solved state can occur by simple,dissolution of the carrier or by the complex,microbial process,of oxidative,degradation.,The,re-