Mass spectrometry showed the new amino acid from diatom cell walls to be a dihydroxyproline. X-ray analysis of the unmarked free amino acid by the use of the symbolic addition procedure revealed the complete three-dimensional structure as 2,3-cis-3,4-trans-3,4-dihydroxy-L-proline with carbon number 3 being 0.06 angstrom above the plane of the pyrrolidine ring.