Ditylum brightwellii grown on NO2- as a nitrogen source took up and assimilated NO2- only in the light, apparently via a photosynthetic nitrite reductase. Assimilation was inhibited by dichlorophenyldimethylurea (DCMU), KCN, partially by 2,4 dinitrophenol, and by NO3-. Kinetics of inhibition of NO2- assimilation by NO3- appeared to be “competitive.” D. brightwellii cells grown on NO2- took up NO3- in both light and dark and in both cases the uptake was inhibited by p-chloromercuribenzoate, but not by DCMU, KCN, or by NO2-. Most of the NO3- taken up in the dark was recovered unchanged from the cells. However only 40% of NO3- taken up in light was recovered from the cells and no NO2- was found. This suggests that a photosynthetic nitrate reduction mechanism was active in these cells. DCMU inhibited the light-induced NO3- reduction. This mechanism of NO3- reduction is distinct from that involving NADH nitrate reductase in D. brightwellii since the concentration of the latter enzyme is very low in cells grown on NO2-. Saturation kinetics were observed for NO2- and NO3- uptake. Half-saturation concentrations (Ksvalues) were 4 and 2 μM, respectively. These values are compared with those obtained for NO2- and NO3- assimilation by other unicellular algae. The comparisons show lower Ksvalues in oceanic species compared with tide-pool or freshwater algae and they support the idea that Ksvalues for NO3- assimilation may provide a key to understanding species succession when this is due to declining: nitrate concentrations in the sea.