The short-term interaction between nitrate and ammonium uptake was examined for a marine diatom, Thalassiosira pseudonana, grown in turbidostat cultures with excess nitrate ranging from light saturation to light limitation (150, 17, 9 and 3 μmol photons m −2 s −1 for L 150, L 17, L 9 and L 3 cultures, respectively). Nitrate uptake in the absence and presence of ammonium, and ammonium uptake in the absence and presence of nitrate were measured during a 6-min time course for cells from these cultures using the cells-on-filter technique. The effect of ammonium on nitrate uptake was light-dependent. In the control (light saturated, L 150), nitrate uptake with ammonium was only 30% of that without ammonium. In a moderate light-limited culture (L 9), nitrate uptake with ammonium was 38% of that without ammonium. In the most severe light-limited culture (L 3), the depression of nitrate uptake by ammonium disappeared. The lack of the depression of nitrate uptake by ammonium at low irradiance is a possible acclimation to satisfy the increased nitrogen requirement (higher N quota) for these cells grown under low light conditions. This acclimation would be advantageous for phytoplankton living near the nutricline, since nitrate and ammonium would be taken up simultaneously. There was an effect of light and nitrate on ammonium uptake. The ammonium uptake rate without nitrate increased with growth irradiance and with ammonium concentration. The V max values for ammonium uptake without nitrate were 300, 320, 350 and 620 mmol (liter cell vol.) −1 h −1 for L 3, L 9, L 17 and L 150 cultures, respectively. There was a small but significant depression of ammonium uptake by nitrate at high light.