The preferential uptake of selenite (Se(Ⅳ)) over selenate (Se(Ⅵ)) by the marine phytoplankters Chaetoceros calcitrans and Chlorella vulgaris was demonstrated. The Se(Ⅳ)/Se(Ⅵ) ratio in culture medium decreased 3-4 times as the algae grew. The addition of 3.2nM Se(Ⅳ) to the medium was mostly removed by the algae, whereas the higher addition (～6nM) of Se(Ⅵ) remained constant during algal growth. The decrease in Se(Ⅳ) from the medium was greater than the amount of Se taken up by the algae. This difference corresponded approximately to the increase in dissolved organic Se in the culture medium. The characterization of this dissolved organic Se produced by cultures is an important next step to understanding its possible role in nature.