Stable isotope incorporation studies with [1,2-(13)C(2)]acetate, [1,2-(13)C(2),(18)O(1)]acetate, and [2-(13)C]mevalonolactone have been used to investigate the biosynthesis of the terpenoids albicanyl acetate (1), cadlinaldehyde (2), and luteone (3) by the dorid nudibranch Cadlina luteomarginata. The results have shown that all three terpenoids are synthesized de novo by C. luteomarginata and the incorporation patterns are consistent with the biogenetic proposal that the new cadlinalane and luteane carbon skeletons are formed by degradation of a sesterterpenoid precursor. This represents the first demonstration of sesterterpenoid biosynthesis by a marine mollusc. Quantitative analysis has shown that only a small turnover of metabolites takes place during the feeding experiments but that the newly formed molecules have extremely high levels of incorporation of labeled precursors.