We present a method for high-sensitivity nitrogen isotopic analysis of particulate organic nitrogen (PON) in seawater and freshwater, for the purpose of determining the aquatic nitrogen fixation rate through the 15N2 tracer technique for samples that contain a low abundance of organisms. The method is composed of the traditional oxidation/reduction methods, such as the oxidation of PON to nitrate (NO) using persulfate, the reduction of NO to nitrite (NO) using spongy cadmium, and further reduction of NO to nitrous oxide (N2O) using sodium azide. Then, N2O is purged from the water and trapped cryogenically with subsequent release into a gas chromatography column to analyze the stable nitrogen isotopic composition using continuous-flow isotope ratio mass spectrometry (CF-IRMS) by simultaneously monitoring the NO+ ion currents at masses 30, 31, and 32. The nitrogen isotopic fractionation was consistent within each batch of analysis. The standard deviation of sample measurements was less than 0.3‰ for samples containing PON of more than 50 nmolN, and 0.5‰ for those of more than 20 nmolN, by subtracting the contribution of blank nitrogen, 8 ± 2 nmol at final N2O. By using this method, we can determine δ15N for lower quantities of PON better than by other methods, so we can reduce the quantities of water samples needed for incubation to determine the nitrogen fixation rate. In addition, we can expand the method to determine the nitrogen isotopic composition of organic nitrogen in general, such as that of total dissolved nitrogen (TDN; sum of NO, NO, ammonium, and DON), by applying the method to filtrates. Copyright © 2008 John Wiley & Sons, Ltd.