Batch cultures of Trichodesmium erythraeum, strain IMS101, and Crocosphaera watsonii, strain WH8501, were grown under metal- and vitamin-replete conditions to evaluate differences in diazotroph abundance and N-2 fixation rates as well as biomass C:N:P ratios resulting from changes in the concentrations of nitrate (NO3-) and phosphate (PO43-) in culture media. Holding light levels and temperature constant, variations in culture NO3- and PO43- concentrations included (N:P ratios in mu M) 0:0.5, 5:1, 8:0.5, and 16:1. The abundance of both diazotrophs was greatest in the 16:1 and 5:1 N:P ratio treatments (i.e. those grown with 1 mu M PO43-) while the highest N-2 fixation rates for both diazotrophs were observed in the 0:0.5 treatment (i.e. those grown in NO3--free media). Measurable but reduced (similar to 25 to 50% of the rates in cultures grown with no NO3-) N-2 fixation rates were evident in both T. erythraeum and C. watsonii cultures grown with up to 16 mu M NO3-. These results indicate that while diazotrophs grown in the presence of NO3- have significantly lower N-2 fixation rates than those not chronically exposed to NO3-, these lower per cell N-2 fixation rates are compensated for by a greater abundance of diazotrophs in treatments with 1 mu M PO43- and result in comparable volume-integrated rates of N-2 fixation. Additionally, N-2 fixation rates for T. erythraeum and C. watsonii were comparable when normalized to carbon (biomass). Finally, the exponential-phase C:N:P biomass ratios of both diazotrophs were similar to each other as well as to previous studies and varied little among the treatments but increased, often significantly, between exponential and stationary growth phases.