Fixed nitrogen is one of the main limiting nutrients for primary production in the ocean(1-3), where it is biologically available in the form of dissolved inorganic and organic matter. Inorganic nitrogen concentrations are consequently very low in surface waters of temperate ocean gyres, yet fixed nitrogen persists in the form of dissolved organic matter, The small, rapidly cycling organic compounds fundamental to microbial and planktonic growth (such as free amino acids, amines and urea(4,5)) account for only a minor fraction of total dissolved organic nitrogen (DON). In contrast, the vast majority of DON, especially in the deep ocean, resides the form of nitrogenous substances that are resistant to biological degradation, These substances, which represent an enormous reservoir of fixed nitrogen, are not readily identified by conventional biochemical techniques, but have been assumed to consist largely of structurally complex macromolecules resulting from the degradation and spontaneous abiotic condensation of biochemical. precursors(6). Here we present N-15 NMR measurements that contradict this view. Our results show that most higher-molecular-weight DON in the ocean exists in amide form, rather than as a collection of nitrogen heterocycles that might be indicative of spontaneous condensation products. Because these amides are unlikely to form abiotically, the bulk of the ocean’s DON reservoir appears to derive directly from degradation-resistant biomolecules.