Animals of hydrothermal vents live in a unique environment that conceivably could lead to modifications of the usual phosphorus functional groups of importance in living systems. To explore this possibility, specimens of a sea anemone (unidentified) from the TAG hydrothermal field, Mid-Atlantic Ridge, the mussel Bathymodiolus N. sp. from the Mid-Atlantic Ridge, and the tubeworm Riftia pachyptila from the East Pacific Rise were analyzed for compounds containing the carbon phosphorus bond. The analysis was based on the use of 3 1 P-nuclear magnetic resonance, which gives signals for C-P compounds that are well separated from those of biological phosphoric acid derivatives. The animals were extracted to provide a lipid- and a water-soluble fraction, leaving an insoluble, largely proteinaceous solid residue. The lipid and residue fractions were subjected to hydrolysis to release bound forms of phosphonic acids. All fractions were analyzed by 3 1 P-NMR. Aminophosphonic acids [primarily NH 2 CH 2 CH 2 PO(OH) 2 (1) and CH 3 NHCH 2 CH 2 PO(OH) 2 (2)] represented the only type of C-P compound detected. These are well-known constituents of coastal invertebrates. For the mussel and sea anemone, these compounds were present in bound form in both the lipid and insoluble residue. The tube worm contained C-P material only in the insoluble residue, but in quite small amounts. The 3 1 P-NMR method is especially valuable in being able to discriminate between compounds 1 and 2. By this technique, two coastal sea anemones (Tealia felina and Bunadosoma cavernata), previously thought to have 1 as the dominant aminophosphonic acid, were in fact found to be much richer in originally undetected 2. This compound was also detected for the first time in a mussel (Genkensia demissa).