Diazotrophic cyanobacteria can take up combined nitrogen (nitrate, ammonium, amino acids, dissolved organic nitrogen) from solution, but the interaction between N2 fixation and uptake of combined nitrogen is not well understood. We studied the effects of combined nitrogen ) additions on N2 fixation rates in the cyanobacterium Trichodesmium erythraeum (IMS-101) maintained in continuous culture in an N-free medium (YBCII) and a 12:12-h light:dark cycle. We measured acetylene reduction rates, nutrient concentrations, and biomass throughout the 12 h of illumination after the addition of nitrate (0.5–20 μM) at the start of the light period. Compared with unamended controls, Trichodesmium showed strong inhibition of acetylene reduction (up to 70%) in the presence of , with apparent saturation of the inhibition effect at an initial concentration of approximately 10 μM. The inhibition of acetylene reduction persisted through much of the light period as concentration in the culture vessel decreased. Recovery of N2 fixation was observed late in the light period in cultures amended with low concentrations of (<5 μM) when ambient concentrations had decreased to 0.3–0.4 μM in the culture vessel. Nitrate uptake accounted for as much as 86% of total N uptake and, at the higher treatment concentrations, more than made up for the observed decrease in N2 fixation rates. We conclude that Trichodesmium can obtain significant quantities of N through uptake of nitrate and does so in preference to N2 fixation when sufficient is available.