We measured the nitrogen (N) isotopic composition (d15N) of a large set (n 5 38) of high-molecular-weight (HMW) dissolved organic nitrogen (DON) samples isolated from the tropical and subtropical North Atlantic and Pacific Oceans. The d15N signature of surface HMW DON is relatively invariable in both oligotrophic basins (4.1 6 0.6 in the Atlantic; 5.4 6 0.8 in the Pacific) and shows little correlation with sources or concentrations of N supporting new production in the euphotic zone. While large variations in d15N of bulk HMW DON are not apparent, d15N of proteins isolated from sites with relatively high rates of N2 fixation (.80 m mol Nm 22 d21) were consistently depleted in 15N relative to bulk HMW DON and to proteins isolated from sites where N2 fixation does not routinely occur. This small component of HMW DON appears to be cycling more rapidly than bulk HMW DON and may be indicative of fresh DON contributed by organisms in the surface ocean. Furthermore, d15N of DNA extracted from the bacterial size fraction (0.2-0.5 mm) revealed that free-living bacteria may be an important sink for isotopically depleted N produced during N2 fixation. We suggest that there exists a tight coupling between the production and uptake of DON contributed by diazotrophs (N2 fixers) in regions where N2 fixation provides a major input of new nitrogen.