A new sesquiterpenoid coumarin, marmaricin, C₂₄H₃₀O₄, was isolated from the roots of Ferula marmarica L., grown in Egypt. The carbon skeleton of the sesquiterpenoid moiety was identified by selenium dehydrogenation, yielding 1,2,5,6-tetramethylnaphthalene. The coumarin moiety was determined by acid cleavage to give umbelliferone. The structure of marmaricin was assigned on the basis of spectral and chemical evidence.