Max-Planck-Institute for Chemical Ecology, Hans-Knoll-Str. 8, D-07745 Jena, Germany. pohnert@ice.mpg.de We describe a short synthesis of the unusual polyunsaturated 6Z,9Z,12Z,15-hexadecatetraenoic acid found in marine and fresh water diatoms. Using a one pot reductive bis-Wittig-olefination, the homoconjugated tetraene backbone of the fatty acid can be generated from easy available precursors. Reductive olefination allows the non-statistical dissymmetrisation of a symmetrical bis-Wittig salt as key synthon. This short sequence was also applied to the generation of the corresponding 9,10-[(2)H(2)] labeled fatty acid. If administered to cell fragments of Thalassiosira rotula 9,10-[(2)H(2)]-6Z,9Z,12Z,15-hexadecatetraenoic acid is transformed oxidatively to the aldehyde 1,2-[(2)H(2)]-2E,4E/Z,7-octatrienal which is involved in the chemical defense of this alga. Using the synthetic standard it could be shown that the C16:4 omega1 fatty acid is released upon wounding of T. rotula cells. The synthesis with the labeled bis-Wittig salt is of general use and can also be applied for the fast generation of other internally labeled functionalized and non-functionalized polyunsaturated fatty acids. To our knowledge this represents the first synthesis of 6Z,9Z,12Z,15-hexadecatetraenoic acid.