Bio-conversion of Se was examined by incubating Pseudomonas marina in seawater containing either selenite (Na2SeIVO3) or selenate (Na2SeVIO4). At the concentrations of selenite and selenate used (10−4–10−7 mol/L), the growth of P. marina was not inhibited. Under these conditions, selenite was taken up by P. marina, but selenate was not found to enter the cells. Pseudomonas marina incorporated selenite from filtered seawater into sub-cellular fractions, primarily protein (30–50%) and amino acids (44–70%). When incubated in marine broth, P. marina incorporated 75selenite primarily into protein (up to 75%), with a lesser amount into the amino acid fraction (approximately 25%). Insignificant amounts were associated with the bacterial lipid fraction. SeIV was found in the protein and amino acid fractions within 10 min of incubation in medium containing selenite. In uptake studies the level of SeIV in the incubating medium decreased markedly, corresponding presumably to SeIV entering the bacterial cells from the medium. However, simultaneous measurements of total Se in the medium (gas–liquid chromatography following photooxidation) revealed an increasing amount of non-SeIV species of Se in the medium throughout the same period. These results were interpreted as due to the bioconversion of selenite by P. marina into water-soluble non-SeIV metabolite(s) and their subsequent release back into the medium. Up to 35% of the total Se found in the medium after 24 h can be accounted for by this conversion of SeIV into another oxidation state. Pseudomonas marina is also capable of reducing SeIV to elemental Se; this pathway becomes increasingly evident at higher concentrations of Na2SeO3.