We investigated the contribution of dissolved free amino acids (DFAA) and ammonium (NH;) to the nitrogen demand (N-demand) of bacterioplankton in the subarctic Pacific and the Delaware Estuary. Bacteria were isolated from other microorganisms by gravity-filtration through 0.8 pm filters, and then bacterial abundance and nutrient concentrations were measured over time. During exper~ments lasting between 36 and 130 h, DFAA and N H 2 contributed 51 + 45 % and 64 + 54 % (n = 14) to the estimated N-demand, respectively. In 9 of the 14 experiments, DFAA and NHZ contributed over 90 % of the estimated N-demand, implying that dissolved organic nitrogen aside from DFAA (e.g. dissolved combined amino acids) was not a significant source of nitrogen. Additions of glucose (0.1 to 1.0 pM) increased the contribution of NHZ and DON other than DFAA to the bacterial N-demand. In most cases, measurements of amino acid and NH; uptake are sufficient for estimating bacterial nitrogen use.