The toxigenic diatom Pseudo-nitzschia cuspidata, isolated from the U.S. Pacific Northwest, was examined in unialgal batch cultures to evaluate domoic acid (DA) toxicity and growth as a function of light, N substrate, and growth phase. Experiments conducted at saturating (120 μmol photons · m−2 · s−1) and subsaturating (40 μmol photons · m−2 · s−1) photosynthetic photon flux density (PPFD), demonstrate that P. cuspidata grows significantly faster at the higher PPFD on all three N substrates tested [nitrate (NO3−), ammonium (NH4+), and urea], but neither cellular toxicity nor exponential growth rates were strongly associated with one N source over the other at high PPFD. However, at the lower PPFD, the exponential growth rates were approximately halved, and the cells were significantly more toxic regardless of N substrate. Urea supported significantly faster growth rates, and cellular toxicity varied as a function of N substrate with NO3−-supported cells being significantly more toxic than both NH4+- and urea-supported cells at the low PPFD. Kinetic uptake parameters were determined for another member of the P. pseudodelicatissima complex, P. fryxelliana. After growth of these cells on NO3− they exhibited maximum specific uptake rates (Vmax) of 22.7, 29.9, 8.98 × 10−3 · h−1, half-saturation constants (Ks) of 1.34, 2.14, 0.28 μg-at N · L−1, and affinity values (α) of 17.0, 14.7, 32.5 × 10−3 · h−1/(μg-at N · L−1) for NO3−, NH4+ and urea, respectively. These labo-ratory results demonstrate the capability of P. cuspidata to grow and produce DA on both oxidized and reduced N substrates during both exponential and stationary growth phases, and the uptake kinetic results for the pseudo-cryptic species, P. fryxelliana suggest that reduced N sources from coastal runoff could be important for maintenance of these small pennate diatoms in U.S. west coast blooms, especially during times of low ambient N concentrations.