2-Aminoethylphosphonic acid has been isolated from a hydrolysate of insoluble proteinaceous material of Metridium dianthus in 1.1 percent yield. It appears to be incorporated in the protein structure; when trichloroacetic acid is added to partial hydrolysates, precipitates form from which this compound is released on complete hydrolysis. The phosphonic acid was also detected in hydrolysates of the 70-percent ethanol- and chloroform-soluble fractions of the sea anemone. The total amount isolated accounted for 0.99 percent of the dry weight of the animal.