In the course of an investigation of the phospholipids of the abalone shellfish (Haliotis midae), a ninhydrin-reactive substance was detected which showed properties similar to phosphoryl ethanolamine (e.g., no separation by column chromatography on the cation exchange resin 'Amberlite C.G. 120' type 2 with an eluting buffer of pH 3.25). Hydrolysis with 2 N hydrochloric acid at 120° C for 100 h failed to liberate ethanolamine and phosphoric acid, so this substance was identified as 2-aminoethylphosphonic acid. Horiguchi and Kandatsu were the first investigators to isolate this compound from rumen protozoa and Tetrahymena pyriformis W, and recently it has also been isolated from the sea anemone (Anthopleura elegantissima) in which it occurs free and esterified with glycerol and sphingosine. The abalone is the highest organism in which the presence of this phosphonic acid has so far been demonstrated. The compound was isolated via lipid extraction (chloroform-methanol 2:1 by volume), hydrolysis with 6 N hydrochloric acid, and column chromatography (on 'Zeokarb 225' and 'Dowex 1-X8' resins), yielding 58 mg of crystalline 2-aminoethylphosphonic acid which melted at 284°-286° C and decomposed at 295° C. Identity was confirmed by mixed melting point with authentic material (no depression) and identical infrared spectra (matching the metastable 'a-form' described by Horiguchi and Kandatsu). Column chromatography on silicic acid showed the phosphonic acid is present in the phospholipids of the abalone, likely as part of a sphingolipid (given the phospholipid fraction's high sphingosine content).