Preincubation of detached leaves of Apium graveolens with deuterium labelled [5,5-2 H2]-deoxy-d-xylulose prior to the elicitation of furanocoumarin biosynthesis by jasmonic acid, resulted in high incorporation of a deuterated prenyl segment into all genuine furanocoumarins. Psoralen, xanthotoxin, bergapten and iso-pimpinellin consistently showed at C-3′ of the furan moiety the presence of a single deuterium atom (31% 2 H). With respect to previous studies, that indicated only a very low incorporation of mevalonate into furanocoumarins, our results clearly demonstrate that the prenylation of umbelliferone in A. graveolens is achieved via the novel mevalonate independent pathway. This is the first example of the participation of DOX-derived DMAPP in a prenylation reaction