Abstract The Se nutritional status of a marine diatom, Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, was assessed by measuring the increase in 14C uptake rate, following Se enrichment. When 10 −10 or 10 −6 M Na 2SeO 3 was added to Se-deplete cultures 24 h before the 14C bioassay was conducted, a 25 or 60% increase in 14C uptake occurred, respectively. Shorter exposure times (e.g., 6–12 h) to Se resulted in no stimulation of 14C uptake, relative to control cultures. Following 24 h of exposure, Se stimulated C uptake rates of Se-deplete cultures from 40 to 60%, when uptake was expressed per cell volume or as disintegrations per minute per millilitre of culture, respectively. Recovery from Se starvation was verified by changes in growth rate and morphology of T. pseudonana. These results indicate that an increase in 14C uptake rate, after exposure to Se for ⩾ 24 h, may be a useful bioassay to assess concentrations of biologically available Se in seawater in the ecologically significant range of 10 −11–10 −8 M Se.